TreeType includes a bunch of stuff that don't have regular saplings associated with them, such as mangrove and azalea trees.
Mangrove has a dedicated propagule block with different behaviour than the others, and azalea trees are grown from azalea blocks, which are solid and have different behaviour to saplings.
We may also want to account for crimson and warped 'trees' in TreeType too, although I'm not sure if those belong there or not.
this isn't practical to fully implement right now due to limitations
imposed by the legacy shitfest system we're using. To make stripped
dynamic, we would need to switch the IDs _and_ variant info dynamically,
and I have no idea what bizarre side effects that might have.
this implementation is very rough due to having to hack around lots more MCPE bullshit, and currently doesn't allow dynamic coral types; but it's there. We'll clean this up after 1.13 migration is done.