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.
