Support downloading the composition as MIDI

[youralien]

Summary: A download button was added to the controls, which downloads the notes in the editor as a MIDI file.

In the process of making this work, the following changes were made:

• Needed to update @magenta/music to latest version in order to overcome an error when calling sequenceProtoToMidi (see link)
• Needed to remove uses of babel which was failing at transpiling to ES6 in order for us to use these new @magenta/music package without errors
• Required node version 12 in the package.json engines. This will ensure someone who is running node 16 isn't horribly confused why they cant yarn install (like I initially was)
• Changed some case sensitive imports because typescript on Linux actually cares about some case sensitivity. Ended up just deving on OSX in the end

Tests: Deployed to https://pair-code.github.io/cococo/. It successfully saves the music after Coconet infills several voices