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_SystemInformation.php

@@ -0,0 +1,93 @@
+<?php
+
+/**
+ * Provides information about active and registered instances of Action Scheduler.
+ */
+class ActionScheduler_SystemInformation {
+	/**
+	 * Returns information about the plugin or theme which contains the current active version
+	 * of Action Scheduler.
+	 *
+	 * If this cannot be determined, or if Action Scheduler is being loaded via some other
+	 * method, then it will return an empty array. Otherwise, if populated, the array will
+	 * look like the following:
+	 *
+	 *     [
+	 *         'type' => 'plugin', # or 'theme'
+	 *         'name' => 'Name',
+	 *     ]
+	 *
+	 * @return array
+	 */
+	public static function active_source(): array {
+		$plugins      = get_plugins();
+		$plugin_files = array_keys( $plugins );
+
+		foreach ( $plugin_files as $plugin_file ) {
+			$plugin_path = trailingslashit( WP_PLUGIN_DIR ) . dirname( $plugin_file );
+			$plugin_file = trailingslashit( WP_PLUGIN_DIR ) . $plugin_file;
+
+			if ( 0 !== strpos( dirname( __DIR__ ), $plugin_path ) ) {
+				continue;
+			}
+
+			$plugin_data = get_plugin_data( $plugin_file );
+
+			if ( ! is_array( $plugin_data ) || empty( $plugin_data['Name'] ) ) {
+				continue;
+			}
+
+			return array(
+				'type' => 'plugin',
+				'name' => $plugin_data['Name'],
+			);
+		}
+
+		$themes = (array) search_theme_directories();
+
+		foreach ( $themes as $slug => $data ) {
+			$needle = trailingslashit( $data['theme_root'] ) . $slug . '/';
+
+			if ( 0 !== strpos( __FILE__, $needle ) ) {
+				continue;
+			}
+
+			$theme = wp_get_theme( $slug );
+
+			if ( ! is_object( $theme ) || ! is_a( $theme, \WP_Theme::class ) ) {
+				continue;
+			}
+
+			return array(
+				'type' => 'theme',
+				// phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
+				'name' => $theme->Name,
+			);
+		}
+
+		return array();
+	}
+
+	/**
+	 * Returns the directory path for the currently active installation of Action Scheduler.
+	 *
+	 * @return string
+	 */
+	public static function active_source_path(): string {
+		return trailingslashit( dirname( __DIR__ ) );
+	}
+
+	/**
+	 * Get registered sources.
+	 *
+	 * It is not always possible to obtain this information. For instance, if earlier versions (<=3.9.0) of
+	 * Action Scheduler register themselves first, then the necessary data about registered sources will
+	 * not be available.
+	 *
+	 * @return array<string, string>
+	 */
+	public static function get_sources() {
+		$versions = ActionScheduler_Versions::instance();
+		return method_exists( $versions, 'get_sources' ) ? $versions->get_sources() : array();
+	}
+}