Better Wither Skeleton Grinding

makes Wither Skeleton Skull grinding more like grinding for Eyes of Ender.

Wither Skeletons now always drops a new item Wither Essence, that can be used to craft Wither Skeleton Skulls.

I honestly never battled the Wither legitly because I hate grinding for Wither Skeleton Skulls, it's the main reason I made this mod.