Add a commandline entry point

It would be very useful for CI use and people that don't have C compilers lying around to be able to conveniently grab a pre-compiled binary with `uvx opentype-sanitizer` or `pipx run opentype-sanitizer` etc.

My current hacky workaround is:
```shell
# Install opentype-sanitizer in a venv or something, then
python -c "import ots,sys;sys.exit(ots.sanitize(*sys.argv[1:]).returncode)" *.ttf
```

I tried to add this myself ([commit](https://github.com/daltonmaag/ots-python/commit/6ad52e6707a86f24e9c473277dfbb48d79a5ca55)) since there's already a `__main__.py` to point it at, but I think because it's been a while since there's been a release on this project a fair bit of the build process has rotted and I got lost in it quite quickly. `setup.py`s are not one of my strong points 😅