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

[jcouv]

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 dotnet/docs#4176
Relates to https://github.com/dotnet/csharplang/blob/master/spec/documentation-comments.md

[sharwell]

⚠️ be careful not to change the existing tests when implementing this feature. Some of them will seem wrong and/or conflicting, but they were carefully written to catch some muscle memory regressions.

[joelmartinez]

fwiw, this support was added to mdoc's ecmaxml format last year :)
mono/api-doc-tools#62

[dend]

Another tag that we might need to make sure is properly supported is <threadsafety>.

cc: @joelmartinez @BillWagner