LibWeb/DOM: Fire transition[cancel,start,run,end] events

Author: https://github.com/LucasChollet
Commit: 1c61ccef40
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3024

Libraries/LibWeb/Animations/AnimationEffect.h

@@ -118,6 +118,7 @@ public:
     bool is_in_the_active_phase() const;
     bool is_in_the_idle_phase() const;
 
+    // Keep this enum up to date with CSSTransition::Phase.
     enum class Phase {
         Before,
         Active,