[Tooling] Changed TagHelper attributes to be SpanKind.Code if not string typed.

- Also added tests to validate that non string TagHelper attributes inherit the SpanKind.COde behavior.
- Removed a block wrapping around single markup spans.

/cc @pranavkm 
No, it's essentially `typeof(string).FullName`. It's restricted to be the `FullName` of the type that the attribute abides by.