Keep track of shuffle-positions teleportees.

Introduces a means of "tracking" both the players and the boss itself
during the `shuffle-positions` ability for the sake of _not_ blocking
the teleport event(s) that could potentially be blocked otherwise.

Note that the change introduced in the PlayerTeleportEvent handler is
not strictly necessary. The final block of code _should_ always yield
the same result, as `shuffle-positions` _should_ be working solely on
positions that are all inside the arena region, but the out-of-bounds
check might be delayed, so we don't know for sure. It's just a safety
precaution, but it really shouldn't be necessary.

Fixes #792

changelog.md

@@ -17,6 +17,7 @@ These changes will (most likely) be included in the next version.
 
 ### Fixed
 - MobArena no longer throws errors when handling block explosions on Minecraft 1.21.
+- The `shuffle-positions` ability now correctly shuffles the position of the boss as well if `monster-teleport` is set to `false`.
 
 ## [0.108] - 2024-01-01
 ### Added