It is useful to indicate clearly that a value is read-only, and we have already been doing so for new code for a long time. But in order to actually enforce this, we have to adjust all existing code. Luckily the change is trivial enough to do (thanks to `eslint --fix`), so do it now. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/432>