new rule - useless `@final` decorator on private method

[DetachHead]

Summary

Originally raised at astral-sh/ty#3140

Playground

main.py

from typing import final, override


class Foo:
    @final # no error. should say something like "@final decorator is useless on private methods"
    def __foo(self):
        ...

class Bar(Foo):
    @override
    def __foo(self): # correct error: Method `__foo` is decorated with `@override` but does not override anything
        ...

[ntBre]

cc @AlexWaygood

It's not really clear to me from the ty discussion if Alex and Charlie thought this would be a better fit for Ruff, or if it doesn't seem worth linting against at all. Have you seen this pattern in a real project where the rule would have been helpful? I'm just trying to get a sense for how common this is. It seems quite niche, but I guess I'm not really opposed to it otherwise.