bug#81054: [PATCH] Add comment-prefixed outline heading recognition

> 2. The original patch only set things up, so I added restoration of
> outline-regexp, outline-level and outline-search-function from the saved
> variables when outline-minor-mode is disabled, so toggling the mode off
> doesn't leave the comment regexp behind.

Since this is a new feature separate from the rest of outline.el,
please move all new lines including new variables to the end
of outline.el.  If this will produce warnings for undefined variables
in outline-minor-mode, please also create a new function
like outline--setup-from-comment-regexp but that will be called
on disabling outline-minor-mode.  What would be a good name
opposite to outline--setup-from-comment-regexp?
Maybe outline--restore-from-comment-regexp?
After that your patch could be pushed.