[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.

web/pdf_layer_viewer.js

@@ -94,7 +94,7 @@ class PDFLayerViewer extends BaseTreeViewer {
       element.textContent = this._normalizeTextContent(name);
       return;
     }
-    element.textContent = await this.l10n.get("additional_layers");
+    element.setAttribute("data-l10n-id", "pdfjs-additional-layers");
     element.style.fontStyle = "italic";
   }