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/HTMLFieldSetElement.h

@@ -54,7 +54,14 @@ private:
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
+    virtual bool is_html_fieldset_element() const override { return true; }
+
     GC::Ptr<DOM::HTMLCollection> m_elements;
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLFieldSetElement>() const { return is_html_fieldset_element(); }
+}