Implement feature for mods to define new world types (#7448)

* Mods can now register world types via a new forge registry, and optionally register a settings screen to tweak them.
  * The default world type for dedicated servers and singleplayer world creation is configurable via forge's common config.

src/main/java/net/minecraftforge/registries/GameData.java

@@ -74,6 +74,7 @@ import net.minecraft.world.gen.treedecorator.TreeDecoratorType;
 import net.minecraftforge.common.ForgeTagHandler;
 import net.minecraftforge.common.MinecraftForge;
 import net.minecraftforge.common.loot.GlobalLootModifierSerializer;
+import net.minecraftforge.common.world.ForgeWorldType;
 import net.minecraftforge.event.RegistryEvent;
 import net.minecraftforge.event.RegistryEvent.MissingMappings;
 import net.minecraftforge.fml.ModContainer;
@@ -180,6 +181,7 @@ public class GameData
         // Custom forge registries
         makeRegistry(DATA_SERIALIZERS, DataSerializerEntry.class, 256 /*vanilla space*/, MAX_VARINT).disableSaving().disableOverrides().addCallback(SerializerCallbacks.INSTANCE).create();
         makeRegistry(LOOT_MODIFIER_SERIALIZERS, c(GlobalLootModifierSerializer.class)).disableSaving().disableSync().create();
+        makeRegistry(WORLD_TYPES, ForgeWorldType.class).disableSaving().disableSync().create();
     }
     @SuppressWarnings("unchecked") //Ugly hack to let us pass in a typed Class object. Remove when we remove type specific references.
     private static <T> Class<T> c(Class<?> cls) { return (Class<T>)cls; }