This makes it possible to use another 'kind' of tag helpers, which isn't
possible today.
This also further decouples the tag helper api surface from the default
implementation.
VCTH now have their own 'kind'.
Also improved generation of display names and error messages where it
was coupled to the type name.
- Replaced the magic strings with extension methods that produce the same behavior as the string keys did.
- Added tests to validate new extension methods.
#1307
Ajay Bhargav Baaskaran
Ryan Nowak
N. Taylor Mullen