Add treefmt and re-format codebase

[jaen]

Sets up treefmt for:

• nix with nixfmt-rfc-style,
• Python with ruff (supposedly a flake8 drop-in replacement) and mypy,
• shell scripts with shellcheck and shellfmt,
• GitHub actions withactionlint.

I have followed excludes from https://github.com/nix-community/robotnix/blob/master/scripts/check.sh and fixed any issues (minor type/lint issues in Python). Next step would be probably to re-enable checking this in CI.

This is based on top of #236, so it has to go in first.

[jaen]

Hm, maybe I should add treefmt-nix (and any future development dependencies) using this pattern – https://figsoda.github.io/posts/2023/developing-nix-libraries-with-subflakes/? I haven't tried it before, but it's probably a good idea to try and avoid propagating treefmt to users?

EDIT: nevermind, forgot how error-prone that is — I thin it's ultimately better to suffer a few supwrfluous dependencies than weird corner–cases of NixCpp.

[jaen]

I think this is ultimately ready for review, but will be easier too look at after #236 is merged, as they share commits and I don't really see way to stack PRs against forks in GH, like you can in a single repo.

[jaen]

Back to a draft for now due to tooling discussion in #239 (comment)

[jaen]

Treefmt now resolved those two issues:
numtide/treefmt#316
numtide/treefmt#317
This means it should be good to merge again.