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/actions/ActionScheduler_CanceledAction.php

@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Class ActionScheduler_CanceledAction
+ *
+ * Stored action which was canceled and therefore acts like a finished action but should always return a null schedule,
+ * regardless of schedule passed to its constructor.
+ */
+class ActionScheduler_CanceledAction extends ActionScheduler_FinishedAction {
+
+	/**
+	 * Construct.
+	 *
+	 * @param string                        $hook Action's hook.
+	 * @param array                         $args Action's arguments.
+	 * @param null|ActionScheduler_Schedule $schedule Action's schedule.
+	 * @param string                        $group Action's group.
+	 */
+	public function __construct( $hook, array $args = array(), ?ActionScheduler_Schedule $schedule = null, $group = '' ) {
+		parent::__construct( $hook, $args, $schedule, $group );
+		if ( is_null( $schedule ) ) {
+			$this->set_schedule( new ActionScheduler_NullSchedule() );
+		}
+	}
+}