LibWeb: Add fast_is<T> for UIEvents::MouseEvent

Author: https://github.com/awesomekling
Commit: 7121539576
Pull-request: https://github.com/SerenityOS/serenity/pull/23606

Userland/Libraries/LibWeb/DOM/Event.h

@@ -144,6 +144,11 @@ public:
 
     Vector<JS::Handle<EventTarget>> composed_path() const;
 
+    template<typename T>
+    bool fast_is() const = delete;
+
+    virtual bool is_mouse_event() const { return false; }
+
 protected:
     void initialize_event(String const&, bool, bool);