Add new French AZERTY layout (Azerty NFZ71-300/A)

[vincentremond]

Add support for the new French AZERTY layout (Azerty NFZ71-300/A),

[dispoclavier]

Normally this would be a matter of just adding a manifest.json, but Chrome uses an outdated XKB codebase, hence ChromeOS crashed and rebooted (like in #100) when I tried activating the fr(afnor) layout (added in xkeyboard-config on 2019-06-02).

Adding it in a background.js would also fail due to #135.

The first time, the Chromebook that this was tested on crashed and rebooted once; turned out it failed activating the fr(afnor) layout. Ultimately it must have succeeded activating the non-existent (in ChromeOS) layout since it got stuck in an infinite crash-and-reboot loop and required a power-wash to become usable again.