i3: Normal numeric workspaces on multi outputs

Use normal workspace numbers with the output name as workspace name, but
strip the name in polybar.

See https://github.com/polybar/polybar/pull/3230 for strip-wsnames.

.config/polybar/config

@@ -40,6 +40,7 @@ enable-ipc = true
 type = internal/i3
 pin-workspaces = true
 format = <label-state> <label-mode>
+strip-wsnames = true
 index-sort = true
 wrapping-scroll = false