[Keymap] Romain's Preonic layout (#7067)

[Keymap] Romain's Preonic layout (#7067)

* Add customized preonic layout

* Add a README detailing a bit the differences from the default preonic layout

* Apply suggestions from code review

Co-Authored-By: fauxpark <fauxpark@gmail.com>
* Remove unnecessary end-of-line backslashes

* Import fix for startup sound (thanks @drashna)

* Remove unnecessary layers and keycodes

* Bring back the _QWERTY layer code

Co-Authored-By: noroadsleft <18669334+noroadsleft@users.noreply.github.com>