Compiler: Rationalize nodes, tokens, visitors, walkers, and rewriters, oh my! (#11853)

Razor's syntax model doesn't provide as much separation between syntax
nodes and tokens as Roslyn's. This is due in part to the fact that
Roslyn's model is designed to support two languages, not just one. When
Razor's syntax model was adapted from Roslyn's, some mistakes were made
when modifying it fit a single language. Currently, the base hierarchy
of Razor's syntax model like so:

```mermaid
---
  config:
    class:
      hideEmptyMembersBox: true
---
classDiagram
    SyntaxNode <|-- SyntaxList
    SyntaxNode <|-- RazorSyntaxNode
    RazorSyntaxNode <|-- SyntaxToken
    RazorSyntaxNode <|-- RazorBlockSyntax
    RazorSyntaxNode <|-- `...Add Other RazorSyntaxNodes`
```

A glaring mistake in the class hierarchy above is that `SyntaxToken`
inherits from `RazorSyntaxNode`, which is reserved for Razor
non-terminal syntax nodes. This PR changes `SyntaxToken` to inherit from
`SyntaxNode` and moves several methods intended for non-terminals to
`RazorSyntaxNode. This creates a much more sensible hierarchy:

```mermaid
---
  config:
    class:
      hideEmptyMembersBox: true
---
classDiagram
    SyntaxNode <|-- SyntaxList
    SyntaxNode <|-- SyntaxToken
    SyntaxNode <|-- RazorSyntaxNode
    RazorSyntaxNode <|-- RazorBlockSyntax
    RazorSyntaxNode <|-- `...And Other RazorSyntaxNodes`
```

This PR adds much cleaner separation between `SyntaxNode`, `SyntaxList`
and `SyntaxToken`, and updates the `SyntaxVisitor`, `SyntaxWalker`, and
`SyntaxRewriter` APIs to better match Roslyn's. This removes a lot of
baggage from `SyntaxToken`, making it easier to convert to a struct in
the near future.

----
CI Build:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2706052&view=results
Test Insertion:
https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/635607
Toolset Test Run:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2706053&view=results

Author: DustinCampbell

src/Compiler/Microsoft.AspNetCore.Razor.Language/legacyTest/Legacy/RazorParserTest.cs

@@ -37,7 +37,7 @@ public void ParseMethodCallsParseDocumentOnMarkupParserAndReturnsResults()
         var syntaxTree = parser.Parse(TestRazorSourceDocument.Create("foo @bar baz"));
 
         // Assert
-        var actual = SyntaxNodeSerializer.Serialize(syntaxTree.Root, validateSpanEditHandlers: true);
+        var actual = TestSyntaxSerializer.Serialize(syntaxTree.Root, allowSpanEditHandlers: true);
         AssertEx.AssertEqualToleratingWhitespaceDifferences(expected, actual);
     }
 }

src/Compiler/Microsoft.AspNetCore.Razor.Language/test/IntegrationTests/FindTokenIntegrationTest.cs

@@ -20,6 +20,6 @@ public void EmptyDirective()
 
         var root = codeDocument.GetSyntaxTree().Root;
         var token = root.FindToken(27);
-        AssertEx.Equal("Identifier;[<Missing>];", SyntaxSerializer.Serialize(token).Trim());
+        AssertEx.Equal("Identifier;[<Missing>];", TestSyntaxSerializer.Serialize(token).Trim());
     }
 }

src/Compiler/Microsoft.AspNetCore.Razor.Language/test/Legacy/RazorParserTest.cs

@@ -3,6 +3,7 @@
 
 #nullable disable
 
+using Microsoft.AspNetCore.Razor.Language;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Language.Legacy;
@@ -48,7 +49,7 @@ public void ParseMethodCallsParseDocumentOnMarkupParserAndReturnsResults()
         var syntaxTree = parser.Parse(TestRazorSourceDocument.Create("foo @bar baz"));
 
         // Assert
-        var actual = SyntaxNodeSerializer.Serialize(syntaxTree.Root, validateSpanEditHandlers: false);
+        var actual = TestSyntaxSerializer.Serialize(syntaxTree.Root, allowSpanEditHandlers: false);
         Assert.Equal(expected, actual);
     }
 }