extensionSystem: Add 'enabled' property

Unlike `state` which represents the actual state, the new property
reflects the desired state, i.e. whether an extension should be
enabled according to the settings.

The Extension app will soon use it to improve its handling of the
case where the two mismatch (e.g. extension errors).

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7004

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3073>

js/misc/extensionUtils.js

@@ -30,6 +30,7 @@ export const ExtensionState = {
 const SERIALIZED_PROPERTIES = [
     'type',
     'state',
+    'enabled',
     'path',
     'error',
     'hasPrefs',