helix: Add nixpkgs-fmt as formatter

roles/home/helix.nix

@@ -24,6 +24,13 @@
 
       [editor.indent-guides]
       render = true
+
+    '';
+
+    file.".config/helix/languages.toml".text = ''
+      [[language]]
+      name = "nix"
+      formatter = { command = "nixpkgs-fmt" }
     '';
 
     packages = with pkgs.unstable; [
@@ -37,6 +44,7 @@
       nodePackages.typescript
       nodePackages.svelte-language-server
       swiProlog
+      nixpkgs-fmt
     ];
   };
 }