Merge `setup.cfg` into `pyproject.toml` and use dependency groups

[samcunliffe]

The napari plugin template has got rid of setup.cfg and only has pyproject.toml, so it seems that there's no reason to keep both.

Definition of done

• [ ] setup.cfg file's contents are moved into pyproject.toml.
• [ ] No change to project metadata.
• [ ] Developer (testing, doc,...) dependencies are handled in dependency groups (PEP735)