gjs started to run eslint during its CI a while ago, so there is an
existing rules set we can use as a starting point for our own setup.
As we will adapt those rules to our code base, we don't want those
changes to make it harder to synchronize the copy with future gjs
changes, so include the rules from a separate file rather than using
the configuration directly.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/609
