Add support for @Dependency(MyDependency.self)

[stephencelis]

Now that @Environment supports a shorthand for objects, we should do similar in our library.

This PR allows you to skip the step of extending DependencyValues with a key path and instead rely directly on the dependency key, which usually aligns with the value.

 extension APIClient: DependencyKey {
   var liveValue: Self { … }
 }

-extension DependencyValues {
-  var apiClient: APIClient {
-    get { self[APIClient.self] }
-    set { self[APIClient.self] = newValue }
-  }
-}
-
 class MyModel {
-  @Dependency(\.apiClient) var apiClient
+  @Dependency(APIClient.self) var apiClient

This new API is less discoverable and friendly to autocomplete, but may be appropriate for application code that refers to a singleton type, and can save that bit of boilerplate above.

[stephencelis]

Snuck this feature into the branch, but could extract to a separate PR...