Common/helix: add nixpkgs-fmt (nix formatter)

roles/common/programs/helix.nix

@@ -6,6 +6,7 @@
       enable = true;
       defaultEditor = true;
       extraPackages = with pkgs; [
+        nixpkgs-fmt
         nil
       ];
        settings = {
@@ -21,6 +22,12 @@
           indent-guides.render = true;
         };
       };
+      languages = [
+        {
+          name = "nix";
+          formatter.command = "nixpkgs-fmt";
+        }
+      ];
     };
   };
 }