chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

vendor/action-scheduler/classes/ActionScheduler_Exception.php

@@ -0,0 +1,11 @@
+<?php
+
+/**
+ * ActionScheduler Exception Interface.
+ *
+ * Facilitates catching Exceptions unique to Action Scheduler.
+ *
+ * @package ActionScheduler
+ * @since 2.1.0
+ */
+interface ActionScheduler_Exception {}