LibWeb: Add fast_is<T> for form, embed, image, and object elements

These are used by Window::supported_property_names(), which can get
very hot.

Userland/Libraries/LibWeb/HTML/HTMLObjectElement.h

@@ -51,6 +51,8 @@ public:
 private:
     HTMLObjectElement(DOM::Document&, DOM::QualifiedName);
 
+    virtual bool is_html_object_element() const override { return true; }
+
     virtual void initialize(JS::Realm&) override;
 
     virtual JS::GCPtr<Layout::Node> create_layout_node(NonnullRefPtr<CSS::StyleProperties>) override;
@@ -87,3 +89,8 @@ private:
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLObjectElement>() const { return is_html_object_element(); }
+}