Use PHP 7.4 featers and PHP 8 polyfills

inc/traits/trait-singleton.php

@@ -43,7 +43,7 @@ trait Singleton {
 	 * @since 2.0.0
 	 * @return void
 	 */
-	public function init() {
+	public function init(): void {
 
 		$this->has_parents() && method_exists(get_parent_class($this), 'init') && parent::init();
 	}