nix: add fmt back

Author: debugloop

formatter.nix

@@ -0,0 +1,28 @@
+{
+  pname,
+  pkgs,
+}: let
+  formatter = pkgs.writeShellApplication {
+    name = pname;
+
+    runtimeInputs = [
+      pkgs.alejandra
+    ];
+
+    text = ''
+      if [ $# -eq 0 ]; then
+        set -- "$(git rev-parse --show-toplevel 2>/dev/null || echo .)"
+      fi
+
+      alejandra "$@"
+    '';
+
+    meta = {
+      description = "format your project";
+    };
+  };
+in
+  formatter
+  // {
+    passthru = formatter.passthru;
+  }