Expand analyzer's resolution of Dartdoc comment references

[srawlins]

Currently the set of allowed comment references is very very small. Something like:

• simple identifier (one word)
• prefixed identifier (foo.bar), for an import prefix and library-level element pair, for a named constructor, and for a static method

We'll expand the set to include:

• static method or constructor on a prefixed library element (foo.bar.baz)
• type literal with type arguments (foo<...>)
• constructor tear-offs with type arguments (foo.bar<...>), including constructor on a prefixed library-level element
• function tear-offs with type arguments (foo<...>) for top-level functions and static methods

All identifiers, including type arguments, should be hoverable and jumpable. Dartdoc's comment reference resolution is already separate from analyzer's and supports more expressions; it can be separately updated to match (or use) analyzer's resolution more closely.

This will be accomplished with a new interface, CommentReferableExpression, and a new member on CommentReference, called expression.

Steps to get there:

1. Add new node; update Identifier, PropertyAccess, TypeLiteral, ConstructorReference, FunctionReference to implement new node; add CommentReferableExpression get expression to CommentReference; deprecate CommentReference.newKeyword and CommentReference.identifier; change implementation of CommentReference.identifier to return expression as Identifier.
2. Publish and update clients to use expression.
3. Update resolution to resolve more comment text to resolve to CommentReferences; remove CommentReference.newKeyword and CommentReference.identifier.

[srawlins]

CC @bwilkerson for any comments.

[srawlins]

CC @eernstg who cited the spec in another recent issue about comment reference resolution.

[eernstg]

Right, that would be #56259 (comment).