Add /jalookup command to find a snitch's group by coordinates

This change adds a new command: /jalookup <x> <y> <z> [world].

If a snitch exists at those coordinates, and the user running the command has access to it, the command returns the name of the group that a snitch is registered to.

The primary purpose for this command is to allow a single snitch monitoring bot to be shared by multiple players without sharing their data.