Attempts to let a survivor pick up the item entity with the given itemindex.
Fails silently if the target slot is occupied, the player recently dropped an item, a slot switch is already in progress, the SlashCoItemPickUp hook blocks it, the entity is flagged DONTPICKUP, or an item ITEM.PrePickUp/ITEM.PrePickUpPrimary/ITEM.PrePickUpSecondary callback blocks it.
On success, equips the item via SlashCo.ChangeSurvivorItem and removes the world entity.