Improve typing references for events

[fmigneault]

The library defines multiple types such as FileSystemMovedEvent, DirMovedEvent, etc., but does not employ them for on_moved event handler. Instead, it uses the generic FileSystemEvent, which somewhat defeats the purpose of having the event-specific types.

When defining a derived event handler class using the explicit types as below, mypy will complain that it "violates the Liskov substitution principle" as described in https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides, which also makes the definition of handler with the specific types provided inadequate.

def on_moved(self, event: FileSystemMovedEvent | DirMovedEvent): ...

Would it be possible to adjust the types to the specific events expected by each method? The current workaround is to type: ignore, also defining the purpose of defining the types in the first place.