extensionUtils: Add DISABLING and ENABLING extension states

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>