Remove CSharpKeyword and CSharpLanguageCharacteristics

[333fred]

When we have the actual roslyn APIs available in the razor compiler, we should remove these types and use SyntaxFacts to make these determinations, rather than having razor try to keep up with roslyn.

[DustinCampbell]

There are also several methods on ParserHelpers that should be rationalized and consolidated. It's possible that several of these (like IsIdentifier) really only target C#.

[DustinCampbell]

Another type that should be included in this rationalization effort is CSharpIdentifier