extensions: Provide separate implementations

For the time being this mostly means re-exporting functions
from the shared module. However openPrefs() is now only
available to extensions, and we stop exporting both
getCurrentExtension() and setExtensionManager() to either
extensions or prefs.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2837>

js/extensions/prefs.js

@@ -0,0 +1,7 @@
+export {
+    getSettings,
+    initTranslations,
+    gettext,
+    ngettext,
+    pgettext
+} from './sharedInternals.js';