Tag helpers attribute code generation

- Followed the "decorate" pattern that we use throughout Razor.
- Enabled the CSharpCodeBuilder to decorate the CSharpCodeVisitor which exposes a CSharpTagHelperCodeRenderer which is indirectly used to render attribute values.
- Fixed up some existing classes that just new'd up the CSharpCodeVisitor class to instead take in the decorated version.
- Tested that we can successfully plugin our own TagHelperAttributeCodeGenerator.
- Moved some existing TagHelper test logic into its own base type to enhance code reusability.

#119
very late to check `TagHelperRenderer` for `null` and this is a silent failure.  there's no use case for throwing away the attribute value `TagHelperChunk`.
instead smarten up the property by giving it a backing field and having it throw `ArgumentNullException` in setter if `value` is `null`.