Pocket Door

## Changelog for 1.6.0
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## 1.6.0
- Safer exit: if your way out is blocked (walled in, lava, floor gone) you now come out at the nearest safe spot,
  with a message. Only when nothing within a few blocks works are you sent to spawn.
- Doors on moving contraptions: a Pocket Door on a Create Aeronautics airship (via Sable) or a Valkyrien Skies ship
  now brings you back out on the ship where it is now, facing the right way. No extra mods required; if the ship is
  gone you are sent to spawn with a message. (1.21.1 and 1.20.1; 1.12.2 gets the safer exit only.)
- Existing worlds carry over unchanged.