Refactor index.js to use new export/import API for apps

aleksis/apps/alsijil/assets/index.js

-import CourseBook from './components/coursebook/CourseBook.vue'
-import messages from "./messages.json";
-
-window.router.addRoute({ path: "/app/alsijil/coursebook/:lessonId", component: CourseBook, props: true });
-
-window.i18n.registerLocale(messages);
+export default [
+    { path: "/coursebook/:lessonId", component: () => import("./components/coursebook/CourseBook.vue"), props: true },
+];