Make python buildable with `pip`

This reconfigures the python extension to allow building directly using `pip`: 1. Adds a `sqlite_vec` folder to conform with the flat package layout. 2. Adds a proper `__init__.py` so the package can be imported. 3. Adds minimal `pyproject.toml` and `requirements.txt` for a basic `setuptools` build. 4. Adds a pytest file to quickly validate the extension. Fixes #230