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/HTMLFrameSetElement.h

@@ -24,6 +24,8 @@ public:
 private:
     HTMLFrameSetElement(DOM::Document&, DOM::QualifiedName);
 
+    virtual bool is_html_frameset_element() const override { return true; }
+
     virtual void adjust_computed_style(CSS::ComputedProperties&) override;
 
     virtual void initialize(JS::Realm&) override;
@@ -37,3 +39,8 @@ private:
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLFrameSetElement>() const { return is_html_frameset_element(); }
+}