[ty] Implement `typing.override` (#21627)
## Summary
Part of https://github.com/astral-sh/ty/issues/155. This implements the
basic check (`@override`-decorated methods should override things!), but
not the strict check specified in
https://typing.python.org/en/latest/spec/class-compat.html#strict-enforcement-per-project,
which should be a separate error code.
## Test Plan
mdtests and snapshots
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Co-authored-by: Carl Meyer <carl@astral.sh>
