Initial implementation for entities, biomes, items, and potions.

This PR introduces the following implementation for the API

* Living Monsters
* Living Animals
* BiomeType
* BiomeTypes
* EntityType
* EntityTypes
* HorseColors
* HorseStyles
* HorseVariants
* ItemType
* PotionType
* PotionTypes
* SkeletonType
* SkeletonTypes

The mixin package structure has also been changed to match the API package structure.

Note: To avoid compiler errors, you must refresh your workspace source by running
'gradlew setupDecompWorkspace --refreshDependencies'
This will apply the latest AT changes in sponge_at.cfg to the source.