[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>

crates/ty_python_semantic/resources/mdtest/narrow/assignment.md

@@ -253,6 +253,7 @@ does["not"]["exist"] = 0
 reveal_type(does["not"]["exist"])  # revealed: Unknown
 
 non_subscriptable = 1
+# error: [invalid-assignment]
 non_subscriptable[0] = 0
 # error: [non-subscriptable]
 reveal_type(non_subscriptable[0])  # revealed: Unknown
@@ -317,7 +318,7 @@ def f(c: C, s: str):
     reveal_type(c.x)  # revealed: int | None
     s = c.x  # error: [invalid-assignment]
 
-    # TODO: This assignment is invalid and should result in an error.
+    # error: [invalid-assignment] "Method `__setitem__` of type `Overload[(key: SupportsIndex, value: int, /) -> None, (key: slice[Any, Any, Any], value: Iterable[int], /) -> None]` cannot be called with a key of type `Literal[0]` and a value of type `str` on object of type `list[int]`"
     c.l[0] = s
     reveal_type(c.l[0])  # revealed: int
 ```