workspace: Improve workspace_next_name.
workspace: Improve workspace_next_name.

This function looks for bound commands that start with `workspace` (ie.
the commands that change to a static workspace) and fetches the
workspace name.

However, if it's actually a list of commands, then the parsing will pick
up the delimiter ("," or ";") and also fail to recognize keywords
("next" etc).

This patch fixes that by properly separating with delimiters.

colemickens/sway,christophgysin/sway,ascent12/sway,SirCmpwn/sway,crondog/sway,crondog/sway,sleep-walker/sway,christophgysin/sway,4e554c4c/sway,colemickens/sway,ptMuta/sway,johalun/sway,mikkeloscar/sway,1ace/sway,1ace/sway,sleep-walker/sway,4e554c4c/sway,ascent12/sway,colemickens/sway,taiyu-len/sway,taiyu-len/sway,taiyu-len/sway,ascent12/sway,1ace/sway,mikkeloscar/sway