Add support for href in xmldoc, as we have for cref and langword

cref When typing <see you get an automatic completion with cref=”” and the completion of symbols inside cref also works, and QuickInfo shows cref with proper symbol coloring:

langword If you edit what was typed above, there is also a completion for langword offered, the choices for langword are also offered (abstract, async, etc), and the langwords are displayed specially in QuickInfo.

href But when typing href instead, there is no completion for href="" and when displaying this documentation in the IDE, the href is treated as text (no colorization and no link).

Tagging @mairaw @BillWagner for additional Issue relates to https://github.com/dotnet/docs/issues/4176 Relates to https://github.com/dotnet/csharplang/blob/master/spec/documentation-comments.md