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/HTMLFormElement.h

@@ -90,6 +90,8 @@ public:
 private:
     HTMLFormElement(DOM::Document&, DOM::QualifiedName);
 
+    virtual bool is_html_form_element() const override { return true; }
+
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
@@ -122,3 +124,8 @@ private:
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLFormElement>() const { return is_html_form_element(); }
+}