sysmidi: add initial macOS support for midi input/output

[slimsag]

This is the very very early-stages (i.e. hacky experiment!) midi synthesizer I made over last holiday season and showed in https://www.youtube.com/watch?v=b8WDjaZC1C8

Known issues:

• (merge blocker) Code in general needs much cleanup
• (merge blocker) src/sysmidi/examples/input.zig uses the old sysaudio API for writing the synthesized music out to the audio device, so needs to be upgraded. Or, perhaps just use the current examples/piano to support midi input and move my more complex synth elsewhere.
• (merge blocker) I would like to rewrite the example I showed in that video with a very janky terminal UI to instead be a simple Mach GUI application. Should not be that hard, and likely would make the code much more reasonable/sane.
• Windows support would be nice-to-have, not sure which APIs are best here.
• Linux support would be nice-to-have, not sure which APIs are best here.
• WASM support via WebMIDI would be nice-to-have

• By selecting this checkbox, I agree to license my contributions to this project under the license(s) described in the LICENSE file, and I have the right to do so or have received permission to do so by an employer or client I am producing work for whom has this right.