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b30d97e5 - [ty] Support `__setitem__` and improve `__getitem__` related diagnostics (#19578)

[ty] Support `__setitem__` and improve `__getitem__` related diagnostics (#19578) ## Summary Adds validation to subscript assignment expressions. ```py class Foo: ... class Bar: __setattr__ = None class Baz: def __setitem__(self, index: str, value: int) -> None: pass # We now emit a diagnostic on these statements Foo()[1] = 2 Bar()[1] = 2 Baz()[1] = 2 ``` Also improves error messages on invalid `__getitem__` expressions ## Test Plan Update mdtests and add more to `subscript/instance.md` --------- Co-authored-by: David Peter <sharkdp@users.noreply.github.com> Co-authored-by: David Peter <mail@david-peter.de>