Make snippet generation strongly typed.

[CyrusNajmabadi]

• Use generics to more clearly articulate the type of syntax various snippets create
• Simplify code by using language-specific syntax APIs where available (instead of relying on generic ISyntaxFacts helpers)
• Remove CursorAnnotation since it was functioning as an exact duplicate of FindSnippetAnnotation

This change does not alter the root ISnippetProvider interface or otherwise change the path we would need to take to make these APIs public.