[ty] Ignore and reject annotations on non-name targets (#25324)

## Summary

Like Mypy and Pyright, we now ignore annotations on non-name (i.e.,
subscript and attribute) targets, and emit a diagnostic for the invalid
annotation -- with the exception of annotated assignments on `self` and
class attributes (i.e., receivers).

Closes https://github.com/astral-sh/ty/issues/509.