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/extension.js

@@ -1,7 +1,5 @@
 import {ExtensionBase, GettextWrapper, setExtensionManager} from './sharedInternals.js';
 
-export {gettext, ngettext, pgettext} from './sharedInternals.js';
-
 const {extensionManager} = imports.ui.main;
 setExtensionManager(extensionManager);
 
@@ -24,3 +22,7 @@ export class Extension extends ExtensionBase {
         extensionManager.openExtensionPrefs(this.uuid, '', {});
     }
 }
+
+export const {
+    gettext, ngettext, pgettext,
+} = Extension.defineTranslationFunctions();