[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
split of `ITagHelperTypeResolver` from `ITagHelperDescriptorResolver` does not seem worthwhile.  we can imagine numerous ways users may want to override tag helper resolution.  the current split may be appropriate for a few but that doesn't make it the correct split.  for now a single class, perhaps not even implementing an `interface` (unless we absolutely must provide some extensibility at this point), is sufficient and won't result in too large a class.
to split a few things out, could move `static` methods into a `private static` class at the bottom of this file.  other than that, keep the resolver together