Dev: Cleanup before stable release

[GeigerJ2]

I think before the stable release, we should definitely take some time to clean up the code base. Some tasks that come to mind:

• Reduce the usage of kwargs. While very convenient, it's hard to understand what's going on. I think we should consider which arguments functions require, and be more explicit.
• CI: Move to the aiida-core tech stack, e.g. Ruff
• Fix (or add) type annotations. We could use basedpyright for that, great tool. Though, to be consistent with aiida-core it should actually be mypy, though basedpyright seems to be more strict.
• Proper testing directory structure, as currently all tests are just in top-level test_<something>.py files

Happy to discuss this during the coding week. At the current stage, I'm a bit overwhelmed and not sure where to start.