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/HTMLHeadElement.h

@@ -20,7 +20,13 @@ public:
 private:
     HTMLHeadElement(DOM::Document&, DOM::QualifiedName);
 
+    virtual bool is_html_head_element() const final { return true; }
     virtual void initialize(JS::Realm&) override;
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLHeadElement>() const { return is_html_head_element(); }
+}