We introduced quite a few new strings recently, but during the last few
rounds of updates we didn't see new translations updates becoming
available. I only just now recalled the announcement that Mozilla is
moving from Mercurial to Git, and indeed the Mercurial page at
https://hg.mozilla.org/l10n-central hasn't been updated since July
anymore and that's were we used to pull our translations from.
This commit fixes the issue by changing the URLs to the Mozilla Git
repositories hosted on GitHub instead.
Given that the Fetch API is supported since Node.js 18 we should be able to use it when downloading l10n files, which allows us to simplify the code and to make it fully `async`.
- For the generic viewer we use @fluent/dom and @fluent/bundle
- For the builtin pdf viewer in Firefox, we set a localization url
and then we rely on document.l10n which is a DOMLocalization object.
