terminal-notifier: enable __structuredAttrs

The Nixpkgs manual makes __structuredAttrs the preferred default and
requires it for all new top-level packages, so enable it here too
(https://nixos.org/manual/nixpkgs/stable/#var-stdenv-__structuredAttrs).

Assisted-by: Claude Opus 4.8 (claude-opus-4-8)

pkgs/by-name/te/terminal-notifier/package.nix

@@ -20,6 +20,8 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-Hd9cI3R2nQK2deBb5CBYz4DTHAEcO4vzqtA5qZwa1Ao=";
   };
 
+  __structuredAttrs = true;
+
   strictDeps = true;
 
   nativeBuildInputs = [