Utilize Fluent to format dates in the AnnotationLayer

The `AnnotationLayer` may not display correctly formatted data in PopupAnnotations, especially in the GENERIC viewer, since it's using native methods[1] that depend on the *browser* locale instead of the viewer locale as intended.
With Fluent we're able to improve things since it's got built-in support for formatting dates. Not only does this simplify the JavaScript code slightly, but it also gives the localizer more fine-grained control of the desired output.

Please find additional information here:
 - https://projectfluent.org/fluent/guide/builtins.html
 - https://projectfluent.org/fluent/guide/functions.html

---

[1] `toLocaleDateString`, and `toLocaleTimeString`.

l10n/en-US/viewer.ftl

@@ -283,9 +283,8 @@ pdfjs-rendering-error = An error occurred while rendering the page.
 ## Annotations
 
 # Variables:
-#   $date (Date) - the modification date of the annotation
-#   $time (Time) - the modification time of the annotation
-pdfjs-annotation-date-string = { $date }, { $time }
+#   $dateObj (Date) - the modification date and time of the annotation
+pdfjs-annotation-date-time-string = { DATETIME($dateObj, dateStyle: "short", timeStyle: "medium") }
 
 # .alt: This is used as a tooltip.
 # Variables:

src/display/annotation_layer.js

@@ -2242,14 +2242,11 @@ class PopupElement {
       modificationDate.classList.add("popupDate");
       modificationDate.setAttribute(
         "data-l10n-id",
-        "pdfjs-annotation-date-string"
+        "pdfjs-annotation-date-time-string"
       );
       modificationDate.setAttribute(
         "data-l10n-args",
-        JSON.stringify({
-          date: this.#dateObj.toLocaleDateString(),
-          time: this.#dateObj.toLocaleTimeString(),
-        })
+        JSON.stringify({ dateObj: this.#dateObj.valueOf() })
       );
       header.append(modificationDate);
     }