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

llama.el

@@ -352,7 +352,7 @@ expansion, and the looks of this face should hint at that.")
            (prog1 t
              (save-excursion
                (goto-char (match-beginning 0))
-               (when-let (((save-match-data (not (nth 8 (syntax-ppss)))))
+               (when-let ((_(save-match-data (not (nth 8 (syntax-ppss)))))
                           (expr (ignore-errors
                                   (read-positioning-symbols (current-buffer)))))
                  (put-text-property (match-beginning 0) (point)