Create TagHelper specific C# code Generation.

- Added TagHelperChunk generation.
- Added CSharp visitors to understand TagHelperChunks and render corresponding C# code.
- Refactored some code in the CSharpCodeVisitor so it could be utilized in other classes.
- Added a CSharpFieldDeclarationVisitor to render declaration pieces for TagHelper's
- Added metadata to represent specific TagHelper code generation constructs.

#72
Add code generation tests for tag helpers.

#72