wayland: pager: Sort workspaces by coordinates

Suggested here: https://github.com/YaLTeR/niri/pull/1800#issuecomment-3066721683

I'm not sure if this should be an option here, or if the default sorting in libxfce4windowing should be changed, or both. Perhaps it would be a good idea to fill in the workspace geometry on wayland in libxfce4windowing on this occasion...