extensions: Add static defineTranslationFunctions() method

The method can be used to define a set of gettext functions that
call the corresponding method of an extension.

Those functions are very similar to the gettext functions we are
exporting, except that:

 - looking up the extension is delegated to the
   Extension/Preferences class
 - it is possible to avoid examining the stack
   when called with `import.meta.url`

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2838>

js/extensions/extension.js

@@ -1,4 +1,4 @@
-import {ExtensionBase, setExtensionManager} from './sharedInternals.js';
+import {ExtensionBase, GettextWrapper, setExtensionManager} from './sharedInternals.js';
 
 export {gettext, ngettext, pgettext} from './sharedInternals.js';
 
@@ -6,6 +6,11 @@ const {extensionManager} = imports.ui.main;
 setExtensionManager(extensionManager);
 
 export class Extension extends ExtensionBase {
+    static defineTranslationFunctions(url) {
+        const wrapper = new GettextWrapper(this, url);
+        return wrapper.defineTranslationFunctions();
+    }
+
     enable() {
     }