chore: add `py.typed` marker file to support type checking

[sisp]

I've added the PEP 561 py.typed marker file to support type checking. Without it, mypy is raising the following error on imports from ragas:

Skipping analyzing "ragas": module is installed, but missing library stubs or py.typed marker Mypy(import-untyped)
See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports Mypy(note)