Add new Artifact item type

New "Artifact" item type has minimal properties.

It has a description, but cannot be used for anything until it is identified. The GM need only say "Something prevents you from wielding/wear/using this item." In the case of a cursed item, the GM could opt to identify it for the player when they try to use it.

An artifact is tied to any one other item of any type. Identifying the artifact replaces the artifact with that item.

Artifacts have a field which contain the ID of the linked item. The identification process deletes the artifact and brings the Item.ID object into the player's inventory.