Allow multiple cave liquids in a biome definition (#8481)

Allow multiple cave liquids in a biome definition (#8481)

This allows games to specify biome cave liquids and avoid the old
hardcoded behaviour, but preserves the ability to have multiple
cave liquids in one biome, such as lava and water.

When multiple cave liquids are defined by the biome definition,
make each entire cave use a randomly chosen liquid, instead of
every small cave segment using a randomly chosen liquid.

Plus an optimisation:
Don't place nodes if cave liquid is defined as 'air'