Annotate tag()'s tag_mapping parameter

[NorthIsUp]

tag() leaves tag_mapping unannotated in both __init__.py and the new __init__.pyi, so pyright in strict mode reports reportUnknownMemberType on every usaddress.tag(...) call (its signature is (address_string: str, tag_mapping: Unknown | None = None)).

This annotates it as Optional[Mapping[str, str]], matching the docstring ({'OldTag': 'NewTag'}) and the body, which only calls .get() on it. No runtime change.

Checked: mypy usaddress clean, black --check clean, pytest tests 4972 passed.