Add proper type hints to class_decorator and decorator

[smarie]

Ideally we should do the same than #23 for class_decorator and for decorator.

Note that _Decorator might need to change to support decorating Types too. Or, even more ideally, we would have a _FuncDecorator for function_decorator, a _ClassDecorator for class_decorator and a _Decorator = Union[_FuncDecorator, _ClassDecorator] for decorator.

# @class_decorator() is called without options or parenthesis
@overload
def class_decorator(
    enable_stack_introspection: Callable[P, Any],
    custom_disambiguator: Callable[[Any], FirstArgDisambiguation] = ...,
    flat_mode_decorated_name: Optional[str] = ...,
) -> _Decorator[P]: ...

# @class_decorator() is called with options or parenthesis.
@overload
def class_decorator(
    enable_stack_introspection: bool = ...,
    custom_disambiguator: Callable[[Any], FirstArgDisambiguation] = ...,
    flat_mode_decorated_name: Optional[str] = ...,
) -> Callable[[Callable[P, Any]], _Decorator[P]]: ...

Originally posted by @smarie in #23 (comment)

[smarie]

Closing as duplicate - was already a step in #22