Add example for midi input (maybe output too?)

[Jikstra]

This adds an example for a stereo volume fader that can get controlled through a midi value (ignores status/note and always applies value byte).

• [ ] is using a mspc::channel actually realtime safe? Didn't look further into it but i guess it could block if the the previous pushed value didn't get read?
• [ ] currently the u8 (midi, from 0-127) to f32 conversion/mapping is hacky and sometimes has rounding errors not getting 1.0 or 0.0 as values
• [ ] probably add vumeter again and send it through midi out? but how?

[sletz]

I've read the detail of MIDI support, but in case it helps, some documentation here:

• https://faustdoc.grame.fr/manual/midi/
• https://faustdoc.grame.fr/manual/architectures/