extensionSystem: Fix extension enabling/disabling order

extensionSystem: Fix extension enabling/disabling order

Move disabling of unneeded extensions before enabling new ones
in _onEnabledExtensionsChanged(). Previously, conflicting
extensions could simultaneously be active briefly, leading to
unclean states, memory leaks, freezes, or crashes.

The old logic also caused unnecessary disable/enable cycles
for newly requested extensions, wasting resources. Running
the disable step first prevents these issues.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3835>