lisp: Introduce a `lisp-fill-paragraph-as-displayed' variable.

lisp: Introduce a `lisp-fill-paragraph-as-displayed' variable.

Starting with Emacs 28, filling strings now happens in a
narrowed scope, and looses the leading indentation and can cause
the string to extend past the fill-column value.  Introduce
`lisp-fill-paragraph-as-displayed' as a new variable allowing
opting out of this new behavior in specific scenarios (such as
when using the Scheme major mode, say).

* lisp/emacs-lisp/lisp-mode.el
(lisp-fill-paragraph-as-displayed): New variable.
(lisp-fill-paragraph): Honor it, by avoiding the logic narrow to
strings before applying fill-paragraph.
* test/lisp/emacs-lisp/lisp-mode-tests.el
(lisp-fill-paragraph-respects-fill-column): Test it.
(lisp-fill-paragraph-docstring-boundaries): New test, as a
safeguard to avoid regressions.

Fixes: bug#56197
(cherry picked from commit 192355e54af91ad6e7d1343071a749e1ced29400)