Currently there is no indication that an extension had an error except
for the sensitivity of the switch (which may have a different cause).
This is useful information to users, so add a small error indicator
alongside the updates icon and show the actual error in the details.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2337
The Extensions app code is now independent enough from the rest of the code
base to move it to its own subprojects, like we did for the extensions-tool.
This allows for stand-alone builds of the app, which we are about to use
for distributing it as flatpak.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133
