[ty] Introduce opt-in `missing-override-decorator` rule. (#25111)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

This adds a new rule called `missing-override-decorator` that allows
users to opt into `@override` enforcement. The rule is ignored by
default.

When the rule is enabled, ty will report a missing override decorator on
an override of any method other than `__init__`, `__new__`,
`__init_subclass__` or `__post_init__`. This matches the general
strategy taken by other type checkers (see discussion starting
[here](https://github.com/astral-sh/ruff/pull/25111#issuecomment-4501847451)).

Closes https://github.com/astral-sh/ty/issues/155.

## Test Plan

Please see included tests.

Note that my approach to testing this feature departs from precedent in
that, unlike other rules, `missing-override-decorator` is not enabled by
default in our test suite. I think we can and should change this in
follow-ups, but for the moment that choice makes this PR easier to
review by eliminating churn in unrelated tests.