Add a mushroom-block-only configuration

[skerit]

You mentioned in issue #42 that you might add support for using mushroom blocks instead of noteblocks as placeholders. Is that still on the roadmap, somehow?

You also mentioned there that this would remove a vanilla feature (the "shaving" of mushroom sides), but that can be remedied by injecting a getStateForNeighborUpdate mixin which cancels the update in case it's a "modded" block, and allows it otherwise?

Main reason I'm asking this: I'm not using PolyMC because I want vanilla players to be able to join my modded server, I actually want to add a few custom blocks to my vanilla server and using PolyMC for this seems like the easiest solution. I can program my custom blocks as if they're any basic Fabric block and let PolyMC worry about the rest.

In addition to this, I'd like to keep noteblock & leaf states as they are, so people can still see which instrument & note is actually set on a noteblock.

[skerit]

I made a little attempt at implementing this myself, it wasn't that hard thanks to the way you can filter on states. (I used the FARMLAND filter as an example)