feat(locale): add Azerbaijani (az) translation

[jamalkamaladdin]

This PR adds an Azerbaijani (az) translation.

New file _locales/az/messages.json: 586 keys, all filled in, same key order as _locales/en/messages.json. No other file changed: the extension discovers _locales/*/messages.json on its own, there is no locale registry to update.

[ImprovedTube2]

thank you! @jamalkamaladdin

which ones might still make sense discussing?

• Did you come across difficult / impossible decisions? If so, then,
  ...
  • https://images.google.com and or https://trends.google.com can help a lot too.

(I say this by default to document and appreciate translator's efforts while fresh and remembered)