Beyond The End - Optimization and Fixes

Hide the Possessed Skeleton from JER mob drops

Just Enough Resources lists every registered mob with its loot and has
no option to hide any. The Possessed Skeleton's summoning ritual was
removed from the modpack, but JEI still showed it as a source of
skeleton skulls on JER's "Mob Drops" page.

New client JEI plugin JerHiddenMobs: when the JEI runtime is available
it hides the JER mob pages whose entity is in its HIDDEN list (for now
occultism:possessed_skeleton). JER classes are reached by reflection, so
without JER it does nothing.