Add decorator behavioral tests

src/decorator/README.md

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+# Decorator Test Notes
+
+## Terminology
+
+Some terms are simplified in these tests to make it easier to communicate
+meaning and intent.
+
+- **static** is used to refer to all class elements that are used with the
+  `static` keyword, including fields and methods. This should be thought of as
+  "elements that belong to the class definition".
+
+- **instance** is used to refer to all class elements that are used _without_
+  the static keyword, including fields and methods. This should be thought as
+  "elements that belong to the class instance". Some of these elements are
+  assigned to the class prototype rather than the instance, but from a typical
+  usage perspective, a user will only call these methods when dealing with an
+  instance of the class. Most users do not ever deal directly with the class
+  prototype.
+
+- **methods** refers to all public/private _methods_, _getters_, and _setters_.
+  These are elements that do _not_ have a per-instance initializer, and for this
+  reason they are grouped together for things like initialization and decorator
+  application.
+
+- **fields** refers to all public/private _fields_ and _accessors_. Accessors
+  here refers to auto-accessors created with the `accessor` keyword, _NOT_
+  getters and setters created with the `get` and `set` keywords. These are
+  elements that _have_ an initialization step per-instance, and grouped
+  together for initialization and decorator application for that reason.
+
+- **Extra-initializers** refers to all initializers added via
+  `context.addInitializer`. This distinguishes them from standard initializers
+  for fields and accessors, which run in a different order and with different
+  timing.
+
+## Structure
+
+- Element decorator templates all follow the same basic structure in order to
+  share as much code as possible for repeated tests.
+
+  - The class element being tested is named `element` for public elements and
+    `#element` for private elements.
+
+  - If the element is gettable, there is a `getElement` public method that
+    directly gets the element. If the element is a field, accessor, or getter,
+    this will call any getters and return the value. If it is a method, it will
+    return the method itself. Setters are not gettable.
+
+  - These is a `setElement` public method that directly sets the element. If the
+    element is a field, accessor, or setter, this will call any setters and set
+    the value. Getters and methods are not settable.
+
+- Each element template folder is broken down into the following directory
+  structure:
+
+  ```
+  .
+  └── [element type]/
+      └── [test case type]/
+          ├── private/
+          │   ├── instance/
+          │   │   ├── cls-decl.template
+          │   │   └── cls-expr.template
+          │   └── static/
+          │       ├── cls-decl.template
+          │       └── cls-expr.template
+          └── public/
+              ├── instance/
+              │   ├── cls-decl.template
+              │   └── cls-expr.template
+              └── static/
+                  └── cls-decl.template/
+                  └── cls-expr.template
+  ```
+
+  This structure allows each test to easily select the types of values it
+  applies to and test them across both class declarations and expressions.
+  Private and public are split out because there are some common differences
+  between them (e.g. `context.private`, `context.name`) and the same is true for
+  static vs instance (e.g. `context.static`).
+
+

src/decorator/accessor-deco-invalid-return-get.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator cannot return invalid getter
+template: accessors/error/**/*
+features: [decorators]
+---*/
+
+//- decorators
+function dec() {
+  return {
+    get: 123
+  };
+}
+
+//- application
+@dec
+
+//- error
+TypeError

src/decorator/accessor-deco-invalid-return-init.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator cannot return invalid init
+template: accessors/error/**/*
+features: [decorators]
+---*/
+
+//- decorators
+function dec() {
+  return {
+    init: 123
+  };
+}
+
+//- application
+@dec
+
+//- error
+TypeError

src/decorator/accessor-deco-invalid-return-set.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator cannot return invalid setter
+template: accessors/error/**/*
+features: [decorators]
+---*/
+
+//- decorators
+function dec() {
+  return {
+    set: 123
+  };
+}
+
+//- application
+@dec
+
+//- error
+TypeError

src/decorator/accessor-deco-invalid-return.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator cannot return invalid init
+template: accessors/error/**/*
+features: [decorators]
+---*/
+
+//- decorators
+function dec() {
+  return 123;
+}
+
+//- application
+@dec
+
+//- error
+TypeError

src/decorator/accessor-deco-returns-get.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator can return a new getter
+template: accessors/standard/**/*
+features: [decorators]
+---*/
+
+//- decorators
+function dec({ get }) {
+  return {
+    get() {
+      assert.sameValue(get.call(this), undefined);
+      return 123;
+    }
+  };
+}
+
+//- application
+@dec
+
+//- assertions
+assert.sameValue(classOrInstance.getElement(), 123);

src/decorator/accessor-deco-returns-init.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator can return a new init
+template: accessors/standard/**/*
+features: [decorators]
+---*/
+
+//- decorators
+function dec() {
+  return {
+    init(value) {
+      assert.sameValue(value, undefined);
+      return 123;
+    }
+  };
+}
+
+//- application
+@dec
+
+//- assertions
+assert.sameValue(classOrInstance.getElement(), 123);
+classOrInstance.setElement(456);
+assert.sameValue(classOrInstance.getElement(), 456);

src/decorator/accessor-deco-returns-set.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator can return a new getter
+template: accessors/standard/**/*
+features: [decorators]
+---*/
+
+//- decorators
+function dec({ set }) {
+  return {
+    set(value) {
+      assert.sameValue(value, 123);
+      set.call(this, 456);
+    }
+  };
+}
+
+//- application
+@dec
+
+//- assertions
+assert.sameValue(classOrInstance.getElement(), undefined);
+classOrInstance.setElement(123);
+assert.sameValue(classOrInstance.getElement(), 456);

src/decorator/accessor-with-init-deco-returns-get.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator can return a new getter
+template: accessors/with-init/**/*
+features: [decorators]
+---*/
+
+//- initializer
+123
+
+//- decorators
+function dec({ get }) {
+  return {
+    get() {
+      assert.sameValue(get.call(this), 123);
+      return 456;
+    }
+  };
+}
+
+//- application
+@dec
+
+//- assertions
+assert.sameValue(classOrInstance.getElement(), 456);

src/decorator/accessor-with-init-deco-returns-init.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator can return a new init that chains with default initializer
+template: accessors/with-init/**/*
+features: [decorators]
+---*/
+
+//- initializer
+123
+
+//- decorators
+function dec({ get, set }) {
+  return {
+    init(value) {
+      assert.sameValue(value, 123);
+      return 456;
+    }
+  };
+}
+
+//- application
+@dec
+
+//- assertions
+assert.sameValue(classOrInstance.getElement(), 456);
+classOrInstance.setElement(789);
+assert.sameValue(classOrInstance.getElement(), 789);

src/decorator/accessor-with-init-deco-returns-set.case

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-applydecoratorstoelementdefinition
+desc: Accessor decorator can return a new setter when initializer is present
+template: accessors/with-init/**/*
+features: [decorators]
+---*/
+
+//- initializer
+123
+
+//- decorators
+function dec({ set }) {
+  return {
+    set(value) {
+      assert.sameValue(value, 456);
+      set.call(this, 789);
+    }
+  };
+}
+
+//- application
+@dec
+
+//- assertions
+assert.sameValue(classOrInstance.getElement(), 123);
+classOrInstance.setElement(456);
+assert.sameValue(classOrInstance.getElement(), 789);

src/decorator/accessors/error/private/instance/cls-decl.template

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+path: language/statements/class/decorator/accessor/error/private/instance/
+name: private acessor decorator evaluation error in class declaration
+features: [class, decorators, class-fields-private]
+esid: prod-FieldDefinition
+---*/
+
+/*{ decorators }*/
+
+function evaluate() {
+  class C {
+    /*{ application }*/
+    accessor #element;
+  }
+}
+
+assert.throws(/*{ error }*/, evaluate);

src/decorator/accessors/error/private/instance/cls-expr.template

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+// Copyright (C) 2024 Kristen Maevyn Hewell Garrett. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+path: language/expressions/class/decorator/accessor/error/private/instance/
+name: private acessor decorator behavior in class expression
+features: [class, decorators, class-fields-private]
+esid: prod-FieldDefinition
+---*/
+
+/*{ decorators }*/
+
+function evaluate() {
+  var C = class {
+    /*{ application }*/
+    accessor #element;
+  }
+}
+
+assert.throws(/*{ error }*/, evaluate);
+