Move spellchecking into fmt

[KaiserKarel]

We currently do loads of fixup commits in PRs because of spelling errors due to two reasons:

• nix fmt is great, but misses out on spellchecking. nix build ./#checks.{system}.spellcheck is verbose.
• cspell isn't great at checking rust and golang code, and errors on variable names, function names etc. Ideally we would ignore code and only check doc comments.

It'd be nice if tree-fmt supported spellchecking, and more specifically would set regexes per language to ensure we only check what we care about.

[KaiserKarel]

Tracking adding support for cspell to tree-fmt here