add "location last opened" to the list command

[Twisted-Code]

Chances are, if I'm using the /sophisticatedbackpacks list command, it's because I lost or misplaced a backpack (or else someone with whom I'm playing did likewise). In this case, to avoid duplicates pointing to the same UUID (which I seem to remember has caused race conditions in the past), it would be ideal if I could find the backpack instead of spawning it in, for which knowing the location in which it was last opened (either where I was standing if it was in my inventory, or where it was placed if it was in the world) would be REALLY helpful.