fix(core): type EventHandler should allow boolean Items and infer correctly

resolves #2324

Author: joshuaellis

packages/core/src/types/functions.ts

@@ -82,9 +82,7 @@ type EventHandler<
   TResult extends Readable = any,
   TSource = unknown,
   Item = undefined,
-> = Item extends undefined
-  ? (result: AnimationResult<TResult>, ctrl: TSource, item?: Item) => void
-  : (result: AnimationResult<TResult>, ctrl: TSource, item: Item) => void
+> = (result: AnimationResult<TResult>, ctrl: TSource, item?: Item) => void
 
 /**
  * Called before the first frame of every animation.