Added NBTUtil#getDirectItem to directly edit the NBT of items. This makes it possible to get an NBTItem instance which directly edits the NBT of the specified item without the need to run NBTItem#create.
Added a ClassRegistry to register classes instead of default objects. The main use case of this registry would be to prevent using the Registry with default objects and prevents unwanted usage of those values, as this registry enforces to create new instances.