[Design] Add TagHelperDescriptorResolver implementation.

- Added a basic TagHelperDescriptor creation mechanism to the TagHelperDescriptorResolver.
- Added an ITagHelperTypeResolver to be responsible for determining valid TagHelper types.

#99
- you're enlarging this PR to include a validation we may decide isn't needed in v1
- your placement decisions are leading to an artificial feeling of size that in turn led to an artificial split between two resolution phases
- even if we decide validation makes sense, it might make _more_ sense as a part that's split out