Change to Dungeon Exits Chances

Changed dungeon exits so that they have a minimum 15% chance of exiting
to the Overworld if a dungeon's root dimension isn't the Overworld. Also
made a minor change to the existing special case for the Nether - the
minimum 20% chance only applies if the root dimension isn't the Nether.
Added Config Setting for Limbo Escape

Added a configuration option for toggling whether players can teleport
out of Limbo by walking over Eternal Fabric. This became viable after
Rift Gateways began generating in Limbo again. Keybounce has expressed
an interest in using option to deal with the issue of players returning
home since his Overworld won't have gateways.