DummyHolderSet

A dummy implementation of HolderSet to help with RuntimeResourcePack creation.

In some instances, Minecraft's resource builders expect a HolderSet for specifying a collection of e.g. items. When specifying this collection simply by a TagKey, these must usually be obtained from a net.minecraft.core.Registry. But when creating a RuntimeResourcePack, the only information we need is the actual TagKey itself, which will be serialized to a string beforehand anyway. This class is used for exactly that. It allows us to create a HolderSet from a TagKey without a net.minecraft.core.Registry, and it always serializes to just the tag.