Map workspaces to specific monitors. Left-hand side is the workspace name, and right-hand side is the monitor pattern.
null or (attribute set of (signed integer or string or list of string))
null
{ "1" = 1; "2" = "main"; "3" = "secondary"; "4" = "built-in"; "5" = "^built-in retina display$"; "6" = [ "secondary" "dell" ]; }