feat: Add MIDI constants

[MarquisdeGeek]

Using numbers like 144 for note on, or 38 to switch to a bass guitar, is a little opaque for my tastes so I've added all the names from the General MIDI spec to self-document my generative music.

[Julusian]

I'm open to merging this, but I think the exports of the values need better names.

Perhaps this could exported from midi.js as:

Constants: {
  Instruments,
  Drums,
  Notes,
  Messages,
}

[MarquisdeGeek]

The instrument and drum lists are only valid for GM (General MIDI) instruments, so would need a designation. Would suggest GMDrums, instead of GeneralMIDIDrums to avoid making the names too long?

[Julusian]

I feel like 'GM' doesn't convey enough meaning to explain anything.
Perhaps this would be better added as a comment on the export/documentation, that should be visible to users when looking to use the values

[MarquisdeGeek]

Bump for the above changes, as suggested.