Add new Mapgen V7 floatland implementation (#9296)

Add new Mapgen V7 floatland implementation (#9296)

Floatland structure is vertically-compressed 3D noise.
Uses a lacunarity of 1.618 (the golden ratio) for high quality
noise.
Floatlands appear between user-settable Y limits, with smooth
tapering at each limit.
Simple user-settable density adjustment.
Shadow propagation is disabled in and just below floatlands, no
shadows are cast on the world surface.
Can be reconfigured to create a solid upper world layer between
the Y limits, lakes/seas can be optionally added to this.