Use match case guard to narrow else cases

**Is your feature request related to a problem? Please describe.**
```python
def f(var: str | bool | None) -> None:
match var:
case _ if not var:
reveal_type(var)
case _:
reveal_type(var) # can be narrowed to: str | Literal[True]
```

The case guard `if not var` can be used to narrow `var` for the else case.
This works in mypy `v1.17.1` and it would be great if pyright could support it as well.

The logic seems to be partially implemented already. E.g. with just patterns the else case is narrowed correctly.
```py
def f(var: str | bool | None) -> None:
match var:
case None:
reveal_type(var)
case _:
reveal_type(var) # str | bool
```

--
pyright 1.1.404

Start by reproducing the reported examples with pyright 1.1.404, comparing the guarded match case with the pattern-only case. Trace the existing match-case narrowing behavior and make the else branch account for the `if not var` guard; done when the second `reveal_type(var)` reports `str | Literal[True]`.

Written by the indexing model from the issue text.