Fix Octave double-quoted string line continuations

Fix Octave double-quoted string line continuations

* lisp/progmodes/octave.el (octave-string-continuation-marker): New
defconst after octave-continuation-string.
(octave-continuation-string): Mention it in docstring.
(octave-maybe-insert-continuation-string): Mark unused function as
obsolete.
(octave-help-function): Simplify action.
(octave--indent-new-comment-line): Insert
octave-string-continuation-marker instead of
octave-continuation-string within double-quoted strings (bug#46420).
(octave-indent-new-comment-line):
* etc/NEWS: Describe new behavior.