feat: provide type hints marker (#14)

> Package maintainers who wish to support type checking of their code MUST
> add a marker file named py.typed to their package supporting typing.
> <...>
> For namespace packages (see PEP 420), the py.typed file should be in the
> submodules of the namespace, to avoid conflicts and for clarity.
-- https://peps.python.org/pep-0561/

Author: feeph