[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
Validate `TagHelperAttributeDescriptor`s for all `TagHelperDescriptor`s to ensure that they don't have conflicting attribute types if they target the same tag name and same attrbute name.