refactor: add root level pyproject setup

[jamessizeland]

This is my first PR into this repo so hopefully this is an acceptable way to go about it.

I'm looking at adding a Rust implementation to this project but first I'm familiarizing myself with the existing setup. I noticed that there was a mix of pip and uv use so I've updated the project setup to be a bit more consistent.

This PR:

• adds a top level uv workspace pyproject.toml which allows the quick start to become just uv sync && source .venv/bin/activate.
• replaces the /tests/requirements.txt with its own pyproject.toml that is still pip compatible.
• removes the __init__.py files from the test folders that was adding collisions between the tests when run from the root. (I think this is ok? I can't see anything that it breaks).
• tidies up some of the README setup comments for the new workflow.

This PR does not:

• Unify the pytest configs between the packages, but provides a mostly common setup in the root pyproject.toml.
• Update the CI workflow to use the uv and root level workflow.

Happy to make any suggested changes that better fit in with the project direction.

(This is not a 4k line diff, that's mostly the uv.lock file autogenerated)