Extend the spawn lists to include an "underground" for none underwater mobs

Currently there are 8 "spawners" or spawn lists in the game that are used by biomes to consider what to spawn and how. "monster", "creature", "ambient" have very distinct uses for the spawning mechanics, and it makes sense to separate out mobs that spawn in a different volumes like squids to water. But this has left the "water" variation with an extra typing above the "air" mob spawn lists with "underground".
An issue I can see with this is that Stray spawns only happen on the surface based on a chance placed on skeleton spawns in "icy" type biomes. Stray spawning could be simply made like any other spawn by setting them to the "surface" spawn list as apposed to a chance spawning based on the sun and moon.
This would also open up having mobs that spawn only underground in some biomes, for instance Caves Spiders in Jungle caves. Bat spawning could also be managed by an "underground" spawn list.

And just for fun if we're really extending the spawn lists there could also be a "lava" spawn list for the nether mobs that spawn in lava.