PEP 561 compatibility: Distribute type information

Are there any plans to distribute type information for this library, i.e. becoming [PEP 561](https://peps.python.org/pep-0561/) compatible? There are extensive type hints already, so I'm wondering if adding a `py.typed` file would be feasible.

MyPy is currently complaining about the tokencost library:

> error: Skipping analyzing "tokencost": module is installed, but missing library stubs or py.typed marker [import-untyped]