import correctly rrule lib, and fix reccurent event display

lib/bundles/fullcalendar.js

@@ -29,7 +29,8 @@
  * ---------------------------------------------------------------------
  */
 // RRule dependency
-window.rrule = require('rrule');
+// lib is exported as a module, so it must be imported as a module
+import { } from 'rrule'
 
 // Fullcalendar library
 require('@fullcalendar/core');