formatter: remove script, add treefmt.toml + keep-sorted (#7056)

currently ./format does a bunch of stuff with git that was only necessary for nixfmt but not nixfmt-tree, so I deleted it (just use treefmt command directly instead). I also added keep sorted to keep the really long module lists sorted (already used in nixpkgs).

.github/workflows/test.yml

@@ -23,7 +23,7 @@ jobs:
           exit 1
         fi
     - run: nix-build --show-trace -A docs.jsonModuleMaintainers
-    - run: ./format --ci
+    - run: nix fmt -- --ci
     - run: nix-shell --show-trace . -A install
     - run: yes | home-manager -I home-manager=. uninstall
     - name: Run tests