Use dependency groups and better define the extras

We should:
- [ ] Replace the dev extras with [dependency groups](https://packaging.python.org/en/latest/specifications/dependency-groups/)
- [ ] Define and document the extras we want to expose to users
- [ ] Ensure the extras / dependency groups are in sync between pyproject.toml and pixi.toml (we duplicate the infos and only test pixi.toml)