lint: Import eslint rules from gjs
    
    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.