feat: add module member name sorting to Stubgen

[tlambert03]

hey @wjakob, thanks for the awesome package.

this relates to #899 (but may not be exactly the same thing). My stubs are getting generated out of order, and therefore have Undefined name errors for some default values when running things like ruff on the output, due to inspect.getmembers() sorting members alphabetically.

I recognize this is a tricky topic, so this PR is designed to do nothing by default, but leave a little bit of agency to the end user. It adds a new member_sort parameter to StubGen

class StubGen:
    def __init__(
        self,
        ...
        member_sort: Optional[Callable[[tuple[str, Any]], Any]] | Literal['as-defined'] = None,
    ) -> None:

if a Callable or None (the default) is passed, it is used directly in sorted(getmembers(value), key=self. member_sort)

if the literal string 'as-defined' is passed, then the module members are sorted as they are defined in the module (using the module __dict__).

let me know what you think