[red-knot] Add support for unpacking targets in `with` statement

### Description

For example:

```py
class Unpacked:
    def __enter__(self) -> tuple[int, int]:
        return (1, 2)

    def __exit__(self, exc_type, exc_value, exc_tb):
        return


with Unpacked() as (x, y):
    reveal_type(x)  # revealed: int
    reveal_type(y)  # revealed: int
```