Make artifact swords / knifes usable in sheaths and scabbards

In `artifact_weapon_datum`, the last two entries have the flags "CHOP" / "STAB", they represent `ARTWEAP_SWORD` / `ARTWEAP_KNIFE`. This is now used when loading old saves that only have the flags "CHOP" / "STAB" to add the corresponding SHEAT_* flag to the item type and when creating new artifacts. Note that the code in iuse also checks for a maximal volume, the artifact might fail that check and not be usable there.

Fixes #7301
That's a fixed text, it's part of the description of the item. Including the *current* key binding is possible but not better than just telling the default binding. Once the user changes it, it would be wrong again.

What about not mentioning the key binding and just saying "It may have unknown powers; activate it to find out." Player s should know how to activate an item.