8584b38cfb switched to shellwords for
completion in command-mode. This changes the conditions for choosing
whether to complete the command or use the command's completer.
This change processes the input as shellwords up-front and uses
shellword logic about whitespace to determine whether the command
or argument should be completed.
* Fix shellwords delimiter handling
This allows commands such as `:set statusline.center ["file-type"]` to
work. Before the quotes within the list would mess it up.
Also added a test to ensure correct behavior
* Rename Delimiter -> OnWhitespace