Use move semantics in inventory::add_item()

[BrettDong]

Summary

None

Purpose of change

When profiling #55099 I noticed some excessive item copy construction showing up on the call tree.

Describe the solution

When adding an item to character inventory, move construct rather than copy construct the new item.

Describe alternatives you've considered

Testing

Reduces 2 seconds in starting_items test with this simple change.

Additional context

[mqrause]

If it's this noticeable for starting_items, it should have a significant effect on crafting_inventory, too.