[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 not touching up existing code.  fix things like this in the PR where it's introduced.
- I also pointed out the `tagHelperDescriptorResolver` should be last