What you need is a synthesizer. Godot has support for generated audio streams, but how it is generated is up to users to implement. This is what Bosca uses. It has a custom software synthesizer and it produces data for the Godot’s audio stream to play back.
You can totally make an editor plugin similar in functionality to Bosca Ceoil. You can probably even adapt the app itself into a plugin relatively easily. The app is just a bunch of fancy UI on top of the synthesizer, but you can make whatever you want with the synthesizer. It’s also open source and free to use :)
This creates a very strong argument for not having a music editor as built-in functionality.