Mob AI Tweaks

-Added zombie_pigmen_with_crossbows_local_difficulty config for adjusting the minimum local difficulty needed for zombie pigmen to spawn with crossbows (default is 250% or 2.5)

-Added door zombies. Door zombies hold a wooden door as a shield. They can only be attacked from behind or with any weapon that can bypass shields. They have a 1% chance of spawning if the local difficulty is 2.5 or higher. Only normal adult zombies that do not have a mount can be door zombies.