bug#48724: Code collapsing and outline headings for elisp

[Juri Linkov]

> Currently, outline-minor-mode has the capability to treat "defun" constructs
> as heading, allowing one to choose to hide some code from view by collapsing
> the region of code.  It would be beneficial for the actual major mode itself
> (i.e. emacs-lisp-mode) to handle the code collapsing, leaving the code 
> highlighting
> intact.  Code collapsing could also be extended beyond not only collapsing
> functions and variables ((defun, defvar), but also to additional constructs 
> that could
> be indented (e.g. lists '(...), let, let*, pcase) and other constructs users 
> could
> find useful (e.g. if, when, ).
>
> And for outline-minor-mode, the mode could then be used to further enhance 
> the structuring
> of larger chunks of code (e.g. a collection of functions) using outline 
> headings.  Outlining
> would then be a separate capability different from code collapsing.

Currently everything is customizable in outline-minor-mode by just
two variables: outline-regexp and outline-level.  So when necessary,
emacs-lisp-mode could provide an option that will set different values
of these variables, one of which could set outline-regexp to a regexp
that matches: lists, '(...), let, let*, pcase, if, when, etc…