Simplify the implementation of `typing.Annotated`

[AlexWaygood]

Feature or enhancement

typing.Annotated is currently implemented as a class, but doesn't need to be. All other objects like it in the typing module are implemented as instances of typing._SpecialForm, and we can do the same here. This simplifies the code a lot (making it easier to maintain in the future), and should also be slightly more performant.

Linked PRs

• gh-105509: Simplify the implementation of typing.Annotated #105510