Add basic support for PEP 798

[ilevkivskyi]

Fixes #21489

Things are mostly straightforward. I also add support for old parser. Two non-trivial things:

• I wrap constructor arguments instead of using ARG_STAR actual kind, because there is a limitation in argmap.py that it can't handle structural iterables. We have a bunch of issues open about this, and there were some attempts to fix it, but they stalled.
• I force union inference for star-unpacks in comprehensions. Otherwise we will get too many builtins.object if we use joins. I think it is fine, since this is a new syntax.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅