️🛠 - Add support for processing MIDI messages

[maxwellmattryan]

Description

With the functionality to listen for MIDI messages enabled, it is time to implement the ability for them to actually change the notes of oscillators, start envelopes, etc.

For this to be implemented in its fullest capacity, an envelope generator needs to be implemented as a modulation source. This is because a MIDI message will do two things:

1. Tell an envelope to start / stop its playback
2. Tell an oscillator what frequency to play at

There are a few ways this could be implemented, which are each worth considering:

• Processing buffer(s) of MidiMessage objects into the process / evaluate methods
• Using a MidiNode object that creates a stream on a specific MIDI input port to listen to messages

Requirements

TBD

Checklist

• I have linked the correct milestone and/or project(s)
• I have added the appropriate labels
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