Update the gulp importl10n command to fetch the active language codes

[Snuffleupagus]

• Update the gulp importl10n command to fetch the active language codes

  Rather than having to manually maintain a static list of language codes, it's much easier to simply fetch the active ones from mozilla-central instead.

  As part of this the code in external/importL10n/locales.js was modernized slightly, by using Promises/async functions to get rid of a bunch of annoying callbacks (which shouldn't be a problem for reasonably modern Node.js versions).

• Update l10n files

• Have gulp importl10n list locales which may be candidates for removal

  Rather than having the script remove locales automatically, which seems like a heavy-handed approach at least initially, listing these for manual checking seems nice though.

Smaller diff with https://github.com/mozilla/pdf.js/pull/11213/files?w=1

[timvandermeij]

Good to see this script getting some improvements!

[Snuffleupagus]

As always, thanks for reviewing/merging this (and other PRs)!

With these changes, the following locales are listed as candidates for removal:

Consider removing the following unmaintained locales:
ak, bn-BD, bn-IN, csb, hto, kok, ks, ku, lg, meh, mn, nso, rw, sah, sat, sw, ta-LK, tn, tsz, zam, zu

It looks like most of them haven't been updated for years, but I didn't look too closely and thus wasn't sure if outright removal was the right thing to do in this PR.

[timvandermeij]

I like that it notifies us, but allows us to take action if required. I do think removing those locales would be fine for a follow-up patch.