feat(behaviors): add usb MIDI behavior support

[SolidHal]

adds support for sending MIDI notes across 10 octaves
adds support for sending MIDI control change codes
adds MIDI behavior documentation

docs can be found at docs/docs/behaviors/midi.md

testing:
example implementation on the btrfld keyboard here https://github.com/SolidHal/zmk/tree/btrfld/app/boards/shields/btrfld
tested on both nice_nano_v2 and bluemicro840_v1
verified MIDI output using midisnoop https://linuxaudio.github.io/libremusicproduction/html/tools/midisnoop.html
made a song using this :)

[petejohanson]

Thank you so much for this PR! I've actually been considering MIDI for a silly side project of mine, so it's wonderful timing to set this hit!

Do you have any interest in trying to also get the BLE bits working based on https://midi.org/midi-over-bluetooth-low-energy-ble-midi ?

Will take me a bit to get to a thorough review of this, but super grateful for the PR!

[SolidHal]

@petejohanson glad to hear. i'm hoping this means more people make weird, cool instruments :)

Do you have any interest in trying to also get the BLE bits working based on https://midi.org/midi-over-bluetooth-low-energy-ble-midi ?

perhaps in the future, but not right now. :)

[joelspadin]

FYI, there's now work in progress to add a USB MIDI driver to Zephyr at zephyrproject-rtos/zephyr#81197.