Structurized Reborn

Added support for ListPoolElements, structure elements with multiple variations. Minecraft currently uses this for the Pillager Tower.

Added a ListPool builder to help you generate the ListPoolElement. Here's how one would use the builder to reconstruct the Pillager Tower element:

ListPoolElement lpe = FabricStructurePoolRegistry.ListPool.builder()
            .addStructureElement(new Identifier("minecraft:pillager_outpost/watchtower"))
            .addStructureElement(new Identifier("minecraft:pillager_outpost/watchtower_overgrown"),StructureProcessorLists.OUTPOST_ROT)
            .addProjection(StructurePool.Projection.RIGID)
            .buildListPool();

To register you element to the pool you want to modify, simply call registerList:

FabricStructurePoolRegistry.registerList(new Identifier("minecraft:pillager_outpost/towers"),1,lpe);