bug#58992: 28.2; "lax space matching" no longer works

[Juri Linkov]

> Okay, this is because of:
>
> commit 74d091a0a665da5dc01989d1b06a61ee21b975b2
> Author: Lars Ingebrigtsen <larsi@gnus.org>
> Date:   Fri Sep 10 12:27:28 2021 +0200
>
>     Change the default value of search-whitespace-regexp
>
>     * lisp/isearch.el (search-whitespace-regexp): Change the default
>     to always exclude newlines from the set (bug#21278).  It used to
>     be mode-dependent whether newlines were included or not, and this
>     was confusing as a user interface.
>
> which changed the default value of search-whitespace-regexp from "\\s-+" to
> "[ \t]".

"\\s-+" in many modes includes newlines, so the default could include
explicit newlines as well that it matched in some modes until Emacs 28.