js: Port Config to ESM

The Config module is shared between the main process, D-Bus
services and tests, which previously prevented it from being
ported to ESM.

The previous commit removed the last outstanding blocker, so
we can now port the last remaining module.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2880>

js/ui/sessionMode.js

@@ -9,7 +9,7 @@ import * as Params from '../misc/params.js';
 import {LoginDialog}  from '../gdm/loginDialog.js';
 import {UnlockDialog} from '../ui/unlockDialog.js';
 
-const Config = imports.misc.config;
+import * as Config from '../misc/config.js';
 
 const DEFAULT_MODE = 'restrictive';