This should be ready for review now, I'll take it out of draft once the tests pass. Notes:

• This PR intentionally does not cover documentation or typing.py behavior; I'll leave that to another PR.
• The PEP 695 test suite helped me find some very interesting issues with the symtable effects of this change. I think the current solution should work, but definitely didn't expect that to be the trickiest aspect of the change.
• It's unfortunate that the new AST field is calleddefault_ in Python. It can't bedefault in C because that's a keyword. I may need to change the AST generation machinery to support aliasing a field so the Python-visible name can be just "default".

Will review this weekend

@pablogsal have you had a chance to look at this? The grammar changes should be quite straightforward, so hopefully it's not complicated to review.

I'll spend some time today trying to make it so the field is called "default" not "default_" in Python, because having "default_" in the Python-visible APIs would be ugly.

@ericsnowcurrently the C globals check is failing because this PR now adds a new static type and global singleton,typing.NoDefault (closely analogous tobuiltins.NotImplemented). What do you think we should do? We could simply allowlist the new globals, or somehow makeNoDefault not rely on C globals—but the latter feels like it would be a lot more complicated.

For now let's just whitelist the new singleton, as long as it is stateless and immortal. We can circle back later if the whitelist is a problem.

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