Turn `only` mode into a toggle.

[rbreaves]

Checklist

• [x ] I searched the :help documentation using / for helpful information
• [x ] I have read the frequently asked questions
• [x ] I did not find any similar issues in the issue tracker
• [x ] I prefer making an issue instead of using discussions, Matrix or Telegram

Addition or change
I would like to request that the only mode that essentially fullscreens a pane in my splits to be a toggle when done twice so that it is very simple and easy to switch between a dedicated fullscreen view and going right back to the prior split.

Alternatives considered
Don't want an alternative - just a simple toggle.