flake: add nixfmt formatter

modules/formatter.ulu.nix

@@ -0,0 +1,22 @@
+{ ... }:
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      formatter = pkgs.writeShellApplication {
+        name = "nixfmt-tree";
+        runtimeInputs = [
+          pkgs.nixfmt
+          pkgs.fd
+          pkgs.findutils
+        ];
+        text = ''
+          if [ "$#" -eq 0 ]; then
+            set -- .
+          fi
+          fd --type f --extension nix --hidden --exclude .git --exclude .jj . "$@" \
+            | xargs -r nixfmt
+        '';
+      };
+    };
+}