[ty] Narrow on negative subscript indexing (#22682)
## Summary
Negative subscripts are also indicative of a thing being subcriptable:
```python
class Subscriptable:
def __getitem__(self, key: int) -> int:
return 42
class NotSubscriptable: ...
def _(x: list[Subscriptable | NotSubscriptable]):
if not isinstance(x[-1], NotSubscriptable):
# After narrowing, x[-1] excludes NotSubscriptable, which means subscripting works
reveal_type(x[-1]) # revealed: Subscriptable & ~NotSubscriptable
reveal_type(x[-1][0]) # revealed: int
```
