LibWeb: Add fast_is<T>() for some DOM and layout node subclasses

The generic is<T>() uses dynamic_cast which is fine in the majority
of cases, but when one of them shows up in profiles, we can make it
faster by answering the is-a question manually.

Userland/Libraries/LibWeb/DOM/Node.h

@@ -143,6 +143,9 @@ public:
 
     virtual EventTarget* get_parent(const Event&) override;
 
+    template<typename T>
+    bool fast_is() const = delete;
+
 protected:
     Node(Document&, NodeType);