status/system: Use Intl to format battery percentage

The exact way of formatting a percentage value is locale specific:
https://en.wikipedia.org/wiki/Percent_sign#Correct_style.

We account for that by marking the string for translations, but then
get it wrong for the default US locale (go us!).

But given that the javascript engine itself provides us with a way
of formatting a percentage in a locale-specific way, that seems the
better option than putting the burden on our own developers and
translators.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7005
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2966>

js/ui/status/system.js

@@ -93,8 +93,9 @@ const PowerToggle = GObject.registerClass({
             use_default_fallbacks: false,
         });
 
+        const formatter = new Intl.NumberFormat(undefined, {style: 'percent'});
         this.set({
-            title: _('%d\u2009%%').format(this._proxy.Percentage),
+            title: formatter.format(this._proxy.Percentage / 100),
             fallback_icon_name: this._proxy.IconName,
             gicon,
         });