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_DateTime.php

@@ -0,0 +1,79 @@
+<?php
+
+/**
+ * ActionScheduler DateTime class.
+ *
+ * This is a custom extension to DateTime that
+ */
+class ActionScheduler_DateTime extends DateTime {
+
+	/**
+	 * UTC offset.
+	 *
+	 * Only used when a timezone is not set. When a timezone string is
+	 * used, this will be set to 0.
+	 *
+	 * @var int
+	 */
+	protected $utcOffset = 0; // phpcs:ignore WordPress.NamingConventions.ValidVariableName.PropertyNotSnakeCase
+
+	/**
+	 * Get the unix timestamp of the current object.
+	 *
+	 * Missing in PHP 5.2 so just here so it can be supported consistently.
+	 *
+	 * @return int
+	 */
+	#[\ReturnTypeWillChange]
+	public function getTimestamp() {
+		return method_exists( 'DateTime', 'getTimestamp' ) ? parent::getTimestamp() : $this->format( 'U' );
+	}
+
+	/**
+	 * Set the UTC offset.
+	 *
+	 * This represents a fixed offset instead of a timezone setting.
+	 *
+	 * @param string|int $offset UTC offset value.
+	 */
+	public function setUtcOffset( $offset ) {
+		$this->utcOffset = intval( $offset ); // phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
+	}
+
+	/**
+	 * Returns the timezone offset.
+	 *
+	 * @return int
+	 * @link http://php.net/manual/en/datetime.getoffset.php
+	 */
+	#[\ReturnTypeWillChange]
+	public function getOffset() {
+		return $this->utcOffset ? $this->utcOffset : parent::getOffset(); // phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
+	}
+
+	/**
+	 * Set the TimeZone associated with the DateTime
+	 *
+	 * @param DateTimeZone $timezone Timezone object.
+	 *
+	 * @return static
+	 * @link http://php.net/manual/en/datetime.settimezone.php
+	 */
+	#[\ReturnTypeWillChange]
+	public function setTimezone( $timezone ) {
+		$this->utcOffset = 0; // phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
+		parent::setTimezone( $timezone );
+
+		return $this;
+	}
+
+	/**
+	 * Get the timestamp with the WordPress timezone offset added or subtracted.
+	 *
+	 * @since  3.0.0
+	 * @return int
+	 */
+	public function getOffsetTimestamp() {
+		return $this->getTimestamp() + $this->getOffset();
+	}
+}