- All the other descriptors are already `IEquatable` this should be as well.
- Moved the required attribute descriptor comparer from Legacy => Language. It wasn't in the Legacy folder to begin with so this made sense given that it's already internal.
#1495
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.
Ajay Bhargav Baaskaran
N. Taylor Mullen
Ryan Nowak