Hello @hairpeace0, thanks for this PR! I'm a bit late of 5 years lol
I wanted to merge your PR, after a few small edits. Could you edit your PR / commits, to include the Allow edits from maintainers option. This allows me to do a few modifications before merging:

Useful note for later: http://midi.teragonaudio.com/tech/midispec/ntnoff.htm

All Notes Off
Number: 123

Turns off all notes that were turned on by received Note On messages, and which haven't yet been turned off by respective Note Off messages. This message is not supposed to turn off any notes that the musician is playing on the local keyboard. So, if a device can't distinguish between notes played via its MIDI IN and notes played on the local keyboard, it should not implement All Notes Off. Furthermore, if a device is in Omni On state, it should ignore this message on any channel.

Note: If the device's Hold Pedal controller is on, the notes aren't actually released until the Hold Pedal is turned off. See All Sound Off controller message for turning off the sound of these notes immediately.

@josephernest What exactly do you want to edit. Can't you propose the changes you want to see?

@hairpeace0 I want to merge the first commit only (5a6d187) but not the 2nd and the 3rd.

Indeed adding a second definition file for an instrument (config.txt in addition to already-existing definition.txt) is not wanted. Also we don't want to make a specific definition-file parsing here: c702dd3#diff-12abeff96b5d42b373fde683a1301f9bd00c7f4c06dd7dfc6f2198e2809e915bR333.

How, in Github, to merge only the 1st commit of this PR, but not the 2nd + 3rd @dosas ?

I see two possibilities:

Either the contributor rebases his changes and deletes the 2nd and 3rd commit

If the contributor does not react. You could cherry pick the first commit and create a PR.