LibWeb: Implement immediate execution in HTMLScriptElement preparation

In some cases, Dr. HTML says we should execute the script right away
even if other scripts are running.

Libraries/LibWeb/DOM/HTMLScriptElement.h

@@ -48,6 +48,7 @@ public:
 private:
     void script_became_ready();
     void when_the_script_is_ready(Function<void()>);
+    void execute_script();
 
     WeakPtr<Document> m_parser_document;
     WeakPtr<Document> m_preparation_time_document;