Additional features and minor fixes

Additions and changes:

• Simple Phase Gates will now give slow falling after teleportation for 15 seconds to a player or any other entity if there is empty space below the destination, as a protection measure. (This is an addition to the protection mechanism of Simple Phase Gates, which previously only checked for an obstruction at the destination.)
• unstable_crystal_explodes gamerule was removed and it's functions replaced with a separate command. /unstablecrystalexplode which accepts true or false values. (operator permissions required)

Fixes:

• Interdimensional Gates now also obstruct teleportation through Simple Phase Gates (before only Simple Gates did). Now they prevent it if present at the destination.