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
I only just noticed this is prompting the player to "use a to activate". It might not be 'a' that's bound to that. Consider replacing with "try activating it"?