Add Sentinel support (PEP-661)

[hrolfurgylfa]

Summary

This PR adds support for PEP 661 Sentinel types.

Fixes #3207

Test Plan

I've run this as an LSP for a bit on a bigger private repo and it seems to work, but the main testing was the tests added to test/narrow.rs and test/sentinel.rs.

Some assumptions/things I didn't fully understand and might deserve more attention:

• I'm using NestingContext::toplevel() when creating a Sentinel struct, this is similar to TypeVar/TypeAlias/a few others, I left the same TODO in place as they had. I looked through some of the other types and couldn't find any other way it was being done, so I left it like this for now, but if anyone has any pointers to how it should be done better, please let me know and I can fix it.

• For the cinderx support, I added the sentinel to it in the TODO section of variables that get set to Any. I haven't really diven into what cinderx is and how the support for it is implemented, but if it is important to get it working I can go ahead and look into that.

• I'm using bind_legacy_type_var_or_typing_alias to bind the Sentinel type, I think this should be ok as the PEP is pretty strict in how it can be assigned. I've done some testing and only found one edge case so far that fails because of this, but I don't think Final's should be mixed with Sentinel objects like this anyway, here is the test case that fails:

from typing import Any, Final
from typing_extensions import Sentinel

MISSING: Final[Any]
MISSING = Sentinel("MISSING")

It fails because the Final doesn't notice the assignment, it still thinks the variable was never assigned to.

Status

This PR is mostly ready for review, but I'm keeping it in Draft form until #3562 is addressed, since until then I can't finalize Python 3.15 support.

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