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_NullAction.php

@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Class ActionScheduler_NullAction
+ */
+class ActionScheduler_NullAction extends ActionScheduler_Action {
+
+	/**
+	 * Construct.
+	 *
+	 * @param string                        $hook Action hook.
+	 * @param mixed[]                       $args Action arguments.
+	 * @param null|ActionScheduler_Schedule $schedule Action schedule.
+	 */
+	public function __construct( $hook = '', array $args = array(), ?ActionScheduler_Schedule $schedule = null ) {
+		$this->set_schedule( new ActionScheduler_NullSchedule() );
+	}
+
+	/**
+	 * Execute action.
+	 */
+	public function execute() {
+		// don't execute.
+	}
+}