This is a split from D3995 to make the review easier and because I anticipate that patch to grow even more.
The idea is to make the biomes more (easily) extensible, which was suggested by @smiley D3995#187096.
To showcase the new functionality I rewrote the already fixed rP26003 (currently disabled to make another point)
Compared to D1729 this seems arguably like the more complicated way to adjust biomes on specific maps, but I would think that its scales better.