Add linter/formatter for css

[hmpf]

The actual css that we write ourselves (like themes in DaisyUI 5) ought to look consistent.

So rarely we don't need it in pre-commit, but we must document how to do it.

[johannaengland]

A suggestion from my side would be Biome since it can both format and lint CSS files. And it has integration with pre-commit (if we ever decide we want to add it after all) and GitHub actions