Some item ability stuff

- Two new item abilities that increase damage as sp/hp is lost
- Two new item abilities that reduce damage as sp/hp is lost
- Support for using sp/hp as a weapon's key skill to determine hit chance

rsrc/strings/item-abilities.txt

@@ -12,10 +12,10 @@ Unused
 Weak weapon
 Cause fear
 Call special when attacking
-
-
-
-
+Does less damage as health lost
+Does more damage as health lost
+Does less damage as SP lost
+Does more damage as SP lost