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/schedules/ActionScheduler_Schedule.php

@@ -0,0 +1,22 @@
+<?php
+
+/**
+ * Class ActionScheduler_Schedule
+ */
+interface ActionScheduler_Schedule {
+	/**
+	 * Get the date & time this schedule was created to run, or calculate when it should be run
+	 * after a given date & time.
+	 *
+	 * @param null|DateTime $after Timestamp.
+	 * @return DateTime|null
+	 */
+	public function next( ?DateTime $after = null );
+
+	/**
+	 * Identify the schedule as (not) recurring.
+	 *
+	 * @return bool
+	 */
+	public function is_recurring();
+}