extensionUtils: Add DISABLING and ENABLING extension states

Extensions can export asynchronous enable() and disable()
functions. To guard against re-entrancy when enabling or
disabling an extension, this commit adds two new states:
ENABLING and DISABLING which are set immediately prior
to calling enable() and disable() respectively.

This commit updates the extensions CLI and Extensions app
with new strings for these states.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2364>

js/misc/extensionUtils.js

@@ -38,6 +38,8 @@ var ExtensionState = {
     OUT_OF_DATE: 4,
     DOWNLOADING: 5,
     INITIALIZED: 6,
+    DISABLING: 7,
+    ENABLING: 8,
 
     // Used as an error state for operations on unknown extensions,
     // should never be in a real extensionMeta object.