LibWeb: Add KeyframeEffect IDL objects

Author: https://github.com/mattco98
Commit: 5824916f8c
Pull-request: https://github.com/SerenityOS/serenity/pull/21913

Userland/Libraries/LibWeb/Animations/AnimationEffect.h

@@ -127,6 +127,8 @@ public:
     Optional<double> current_iteration() const;
     Optional<double> transformed_progress() const;
 
+    virtual DOM::Element* target() const { return {}; }
+
 protected:
     AnimationEffect(JS::Realm&);