Implement pattern_type macro

[Veykril]

Does not implement actually type level things though

[Veykril]

Given this is all hidden behind a macro right now I took the liberty of going with a prefix approach for parsing simplicity for now

View changes since the review

[ChayimFriedman2]

This does not support the !null pattern. I'm not sure if extending normal patterns will cause problems, and we need a special parser.

[Veykril]

Yeas it doesn't, though I think !null is not yet used in the standard library 🤦 I even copied a counter-example into minicore. In that case I'd still like to merge the PR as is I think, can be fixed up as a follow up. Once again I wish the project wouldn't dogfood features in the standard libraries..

That does raise the question of what to do about the new type only pattern here. Ideally we would re-use the pattern grammar here as adding a new type-pattern grammar is a lot to deal with.

[rustbot]

☔ The latest upstream changes (possibly #22101) made this pull request unmergeable. Please resolve the merge conflicts.

[ChayimFriedman2]

The problem of using it as normal pattern is that all other places dealing with patterns will need to deal it. I don't personally have an opinion what's better.