Warn when a falsey boolean context is ambigious with `None`

```py
a: bool | None
if not a: # error: the type of 'a' contains "None" and could be a mistake to check it in a boolean context
...
```

Not just bools, but anything that could be false.

The issue names no files or tests. Start by reproducing the shown `bool | None` example and locating the checker’s handling of boolean contexts; done means it warns when a value that may be `None` is tested for falseyness, including non-boolean falsey types.

Written by the indexing model from the issue text.