lightningcss: add mainProgram

pkgs/development/tools/lightningcss/default.nix

···
     changelog = "https://github.com/parcel-bundler/lightningcss/releases/tag/v${version}";
     license = licenses.mpl20;
     maintainers = with maintainers; [ toastal ];
+    mainProgram = "lightningcss";
     # never built on aarch64-linux since first introduction in nixpkgs
     broken = stdenv.isLinux && stdenv.isAarch64;
   };