Add Karabiner Elements (macOS) as software based "firmware"

[peterjc]

e.g. https://blastedbio.blogspot.com/2025/05/what-have-you-done-to-your-keyboard.html (mine) or https://ke-complex-modifications.pqrs.org/#alternative-keyboard-layouts

[precondition]

Feel free to submit a pull request. It would be nice to add more row-staggered keymaps to KeymapDB.

[peterjc]

Thanks. It looks like there isn't a central list of allowed firmware (eg kanata and keyd), so this will be implicit the first time a PR is merged using a new firmware value.