extensions: Replace exported gettext functions

Use the new defineTranslationFunctions() method from the previous
commit to create gettext functions for the module, instead of
re-exporting from the shared module.

It is now up to extension developers to use the more effective

```js
import {Extension} from 'etensions/extension.js';
const {gettext: _} =
    Extension.defineTranslationFunctions(import.meta.url);
```

or the more convenient

```js
import {Extension, gettext} from 'extensions/extension.js';
const _ = gettext;
```

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

js/extensions/prefs.js

@@ -5,8 +5,6 @@ import {extensionManager} from '../extensionsService.js';
 
 setExtensionManager(extensionManager);
 
-export {gettext, ngettext, pgettext} from './sharedInternals.js';
-
 export class ExtensionPreferences extends ExtensionBase {
     static defineTranslationFunctions(url) {
         const wrapper = new GettextWrapper(this, url);
@@ -22,3 +20,7 @@ export class ExtensionPreferences extends ExtensionBase {
         throw new GObject.NotImplementedError();
     }
 }
+
+export const {
+    gettext, ngettext, pgettext,
+} = ExtensionPreferences.defineTranslationFunctions();