workspace_next_name: fallback to next available number
workspace_next_name: fallback to next available number

This changes `workspace_next_name` to use the next available number as
the workspace name instead of the number of outputs. This fixes the case
where a number that is already in use could be returned. The workspace
numbers in use have no relation to the number of outputs so it makes
more sense to use the lowest available number

ascent12/sway,SirCmpwn/sway,1ace/sway,1ace/sway,ascent12/sway,1ace/sway,ascent12/sway