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/abstracts/ActionScheduler_Abstract_RecurringSchedule.php

@@ -0,0 +1,112 @@
+<?php
+
+/**
+ * Class ActionScheduler_Abstract_RecurringSchedule
+ */
+abstract class ActionScheduler_Abstract_RecurringSchedule extends ActionScheduler_Abstract_Schedule {
+
+	/**
+	 * The date & time the first instance of this schedule was setup to run (which may not be this instance).
+	 *
+	 * Schedule objects are attached to an action object. Each schedule stores the run date for that
+	 * object as the start date - @see $this->start - and logic to calculate the next run date after
+	 * that - @see $this->calculate_next(). The $first_date property also keeps a record of when the very
+	 * first instance of this chain of schedules ran.
+	 *
+	 * @var DateTime
+	 */
+	private $first_date = null;
+
+	/**
+	 * Timestamp equivalent of @see $this->first_date
+	 *
+	 * @var int
+	 */
+	protected $first_timestamp = null;
+
+	/**
+	 * The recurrence between each time an action is run using this schedule.
+	 * Used to calculate the start date & time. Can be a number of seconds, in the
+	 * case of ActionScheduler_IntervalSchedule, or a cron expression, as in the
+	 * case of ActionScheduler_CronSchedule. Or something else.
+	 *
+	 * @var mixed
+	 */
+	protected $recurrence;
+
+	/**
+	 * Construct.
+	 *
+	 * @param DateTime      $date The date & time to run the action.
+	 * @param mixed         $recurrence The data used to determine the schedule's recurrence.
+	 * @param DateTime|null $first (Optional) The date & time the first instance of this interval schedule ran. Default null, meaning this is the first instance.
+	 */
+	public function __construct( DateTime $date, $recurrence, ?DateTime $first = null ) {
+		parent::__construct( $date );
+		$this->first_date = empty( $first ) ? $date : $first;
+		$this->recurrence = $recurrence;
+	}
+
+	/**
+	 * Schedule is recurring.
+	 *
+	 * @return bool
+	 */
+	public function is_recurring() {
+		return true;
+	}
+
+	/**
+	 * Get the date & time of the first schedule in this recurring series.
+	 *
+	 * @return DateTime|null
+	 */
+	public function get_first_date() {
+		return clone $this->first_date;
+	}
+
+	/**
+	 * Get the schedule's recurrence.
+	 *
+	 * @return string
+	 */
+	public function get_recurrence() {
+		return $this->recurrence;
+	}
+
+	/**
+	 * For PHP 5.2 compat, since DateTime objects can't be serialized
+	 *
+	 * @return array
+	 */
+	public function __sleep() {
+		$sleep_params          = parent::__sleep();
+		$this->first_timestamp = $this->first_date->getTimestamp();
+		return array_merge(
+			$sleep_params,
+			array(
+				'first_timestamp',
+				'recurrence',
+			)
+		);
+	}
+
+	/**
+	 * Unserialize recurring schedules serialized/stored prior to AS 3.0.0
+	 *
+	 * Prior to Action Scheduler 3.0.0, schedules used different property names to refer
+	 * to equivalent data. For example, ActionScheduler_IntervalSchedule::start_timestamp
+	 * was the same as ActionScheduler_SimpleSchedule::timestamp. This was addressed in
+	 * Action Scheduler 3.0.0, where properties and property names were aligned for better
+	 * inheritance. To maintain backward compatibility with scheduled serialized and stored
+	 * prior to 3.0, we need to correctly map the old property names.
+	 */
+	public function __wakeup() {
+		parent::__wakeup();
+		if ( $this->first_timestamp > 0 ) {
+			$this->first_date = as_get_datetime_object( $this->first_timestamp );
+		} else {
+			$this->first_date = $this->get_date();
+		}
+	}
+}