fix: add treewide formatting with `nixfmt` and others

[gabyx]

@shivaraj-bh:

May I suggest a small PR with a treefmt-nix setup which uses nixfmt + some others like prettier for .md files etc to format treewide everything, such that
other contributors have less pain in their PRs.

• just format
• format check in omnix.nix

Also nixpkgs-fmt is not deterministic enough which is a pain at the moment in the repository, a small line change still might go through the formatter
succesfully, so it is too lenient + outdated.

[srid]

See also #527

[gabyx]

I think since nixfmt now has ignore syntax, I suggest closing it in favor of this issue.

[gabyx]

@srid, @shivaraj-bh : I made a suggestion PR in the dev flake here: #708. Works as a charm. (formatting not yet applied).

This setup would help contributors a lot! Thanks considering this =)