Dungeons: Add nodebox stairs to desert and sandstone dungeons

Dungeons: Add nodebox stairs to desert and sandstone dungeons

Desert and sandstone dungeons have 2 node wide corridors. Previously,
nodebox stairs were disabled because dungeon generation code did not
support nodebox stairs wider than 1 node, now it does.

Add 'stair desert stone' content id to MappgenBasic.

Requires 'mapgen stair desert stone' to be added to Minetest Game.