Add compatibility for _extract_signature_elements to nested typing hints

[lennart-bader]

This commit modifies the _extract_signature_elements method in the DocString class to support nested type hints, e.g., as required when using typing.Callable.

Previously, this resulted in an Exception:
Exception: unexpected nested element after [ while reading param

Example method signature that causes the issue:

def method(self, callback: Callable[[str], None]):
    pass

Instead of tracking a single token (i.e., the [ in this case), the method now uses a stack to track nested tokens.

This pull request addresses #120