Primary Monitor

I didn't find a primary monitor related concept in wayland, maybe on some platforms it doesn't matter, but for desktop systems it is necessary. For this, most synths will use different tools to set up the main display. For example kde has kscreen-doctor. gnome has gnome-randr. However, they are not uniform. For the client, the adaptation work becomes difficult, and the client hopes to have a unified interface to apply it.

I don't think it's a difficult thing to implement Primary Monitor in xdg-output , why don't we do it?