Give Mobs Natural Regen When Playing in Hardcore and Hard Mode

This will make the game harder which will improve the survival aspect.

At the moment no mob has natural regen except for witches, Wither Boss and the Ender Dragon. This makes them easy to beat without much of a challenge. If they could heal it would make the game much for intense.

When you accidentally hit your pet you could feed them to instantly heal them but in case you have no food they should still have a slow regen ability.