LibWeb: Add fast_is<HTMLElement>()

This avoids slow RTTI lookups in selector matching.

Userland/Libraries/LibWeb/HTML/HTMLElement.h

@@ -53,6 +53,8 @@ protected:
     virtual void parse_attribute(FlyString const& name, String const& value) override;
 
 private:
+    virtual bool is_html_element() const final { return true; }
+
     // ^HTML::GlobalEventHandlers
     virtual DOM::EventTarget& global_event_handlers_to_event_target(FlyString const&) override { return *this; }
 
@@ -73,3 +75,8 @@ private:
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLElement>() const { return is_html_element(); }
+}