chore: add license headers (#384)

* feat: check for license headers

* add headers

* format

plugins/notification/src/init.js

@@ -1,3 +1,7 @@
+// Copyright 2019-2023 Tauri Programme within The Commons Conservancy
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: MIT
+
 (function () {
   let permissionSettable = false;
   let permissionValue = "default";