Add TagHelper completions API to TagHelperFactService.

[NTaylorMullen]

Today we expose several APIs that control the binding of TagHelpers to a given HTML element based on the tag name and presence of attributes; we should own the entire completion API.

[rynowak]

The title of this is jankified. This would be a new, completion-focused service.

[NTaylorMullen]

a1cfd22