Override synchronous Process in AnchorTagHelper subclasses

[YoshiRulz]

See #2197 (comment) for explanation.
I've attempted to implement these 1) without simply calling ProcessAsync from Process, which adelikat didn't like, and 2) while minimising code duplication.

[Masterjun3]

I read the comment, but why exactly do we need this? Do we have a specific use case, or is this a "just in case"?

[Masterjun3]

From what I see, some of the asp net core tag helpers also don't override Process. So I would probably close and leave this unmerged if we don't need it.

[YoshiRulz]

It's a "just in case" which I already ran into in practice while writing unit tests.

Looks like the unit tests for ScriptTagHelper are all async. I suspect that they would see odd failures if they made those synchronous for some reason.

[Masterjun3]

I'm in favor of not adding in this complexity if not even asp net core code cares about that.
It only matters to us in the tests (in the other PR), and we're already copying unit test code from the asp net core repository anyway, so that's covered.