Asset unload event moved to public API (#2885)
Asset unload event moved to public API (#2885)

* asset unload event move to public API

* Added example of use of the asset unload event

Co-authored-by: Steven Yau <c4e01c0f87a9d4bf034f0f681553ace272d52b0f@snaphcat.com>
playcanvas/engine,playcanvas/engine,MicroWorldwide/PlayCanvas,MicroWorldwide/PlayCanvas