chore: add py.typed marker file to support type checking (#2029)

I've added the [PEP 561](https://peps.python.org/pep-0561/) `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)
```

Author: sisp