Use _ in all non-binding entries in the varlist of COND-let forms

This was not done until now because doing so results in a warning
because the macro expansion did not actually leave any variables
unused.  This was fixed in Emacs 30.1 and I've added a backport to
Transient, which fixes the issue for that package and packages that
depend on it.

It doesn't remove the warning when compiling this package, but I have
decided I *always* want to use `_' when appropriate, and that I can
live with users seeing warnings, if they do not bother to update to
the latest Emacs release in a timely manner.  Alternatively they can
add the backport early in their init file.

Without consistently using `_' one would always have to count parens
to be sure whether a member of the varlist does or does not bind a
variable.  It is very easy to make mistake when writing or reading
such forms, if `_' is not consistently used in the non-binding cases.

Author: tarsius

elx.el

@@ -1260,7 +1260,7 @@ The value is a cons of the form (FULLNAME . ADDRESS)."
            (when-let*
                ((parts (match-string 2 x))
                 (parts (split-string parts " " t))
-                ((seq-every-p (##string-match-p "\\`\".+\"\\'" %) parts)))
+                (_(seq-every-p (##string-match-p "\\`\".+\"\\'" %) parts)))
              (setq mail (mapconcat (##substring % 1 -1) parts ""))))
           ((string-match (concat "\\(\\S-+@\\S-+\\) "
                                  "[(<]\\(.*\\)[>)]") x)