[api-minor] Move to Fluent for the localization (bug 1858715)

- 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.

external/importL10n/locales.mjs

@@ -70,7 +70,7 @@ function downloadLanguageFiles(root, langCode) {
   const MOZ_CENTRAL_PDFJS_DIR = "/raw-file/default/browser/pdfviewer/";
 
   // Defines which files to download for each language.
-  const files = ["viewer.properties"];
+  const files = ["viewer.ftl"];
   let downloadsLeft = files.length;
 
   const outputDir = path.join(root, langCode);

external/webL10n/README.md

@@ -1,3 +0,0 @@
-The source code for the library can be found at
-
-  https://github.com/fabi1cazenave/webL10n