LibWeb: Add more fast_is<T> helpers for DOM::Node subclasses

Author: https://github.com/awesomekling
Commit: aecb144df0
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4395

Libraries/LibWeb/HTML/HTMLIFrameElement.h

@@ -69,3 +69,8 @@ private:
 void run_iframe_load_event_steps(HTML::HTMLIFrameElement&);
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLIFrameElement>() const { return is_html_iframe_element(); }
+}