[Feature Request] Add user-defined keycodes to REMAP key lists.

[3araht]

It would be wonderful to have a new category dubbed "User-defined," for example, to have user-defined keycodes available for keyboard users.

For example, there are approx. 80 custom keys added to keymap.c for giabalanai keyboard.

Most of them are MIDI Chords.

The goal here is to have the customized keycode names on the REMAP screen, inside the category "User-defined," for example.
Then, REMAP users can choose these user-defined keycodes just like other standard keycodes.

There are two strategies to realize this request:
1, Instead of using "SAFE_RANGE," start user-defined keycodes from a specified keycode number which will be announced by REMAP team.
2, Share user-defined keycode names and those orders as a list via a new dedicated JSON file.

1, is to fix the keycode number.
2, is to feed REMAP what keycodes are defined as user-defined keycodes.

[3araht]

In the JSON file, it would be better to have brief descriptions of each keycode, too.