Add parser support for PEP 696

Ruff's parser does not support the new syntax introduced by [PEP 696](https://peps.python.org/pep-0696/):

```
def f[T=int](): pass
```

I noticed because I'm adding this syntax to CPython and Ruff started failing in CI :).

I saw that you recently switched to a handwritten parser. I'm interested in taking a look at that parser, so I'd like to try to implement this myself.