chore: add py.typed for type checking

[hiro-o918]

Description

Add py.typed marker
https://peps.python.org/pep-0561/#packaging-type-information

Motivation and Context

I want to run type checking on my project with lugi.

Have you tested this? If so, how?

[dlstadther]

I'm not sure if you'll run into issue while type checking. But I'll approve to include the ability.

[hiro-o918]

Thank you for your merging.
In detail, if there is no py.typed, all the classes are handled as Any and no type checking works.
This can makes us write invalid typing as type annotations, because it is not checked by type checkers like mypy