Support VST3

[yupferris]

This is just going to be something we have to do at some point, and I'm convinced that either we migrate to VST3 and break VST2 compatibility (simpler if we ignore old projects) or build our own (hopefully small) abstraction over VST2/VST3 and make both opt-in, but by default build VST3 (I think this is mostly doable).

Assuming we want to migrate to VST3, I've started work on this branch and I think overall it will simplify a lot of things. It's still somewhat early, though, and due to other obligations (and the fact that the docs are currently down right now, ugh) I can't take it much further right now.

[yupferris]

As for old projects, it may be possible to provide a VST2 -> VST3 conversion path, though this would have to be per-DAW. It's likely that the most pragmatic option for this is to continue to support VST2 for a while, but deprecate it, in favor of VST3.