Make lighter for collapsed minor modes customizable (bug#77361)

* lisp/bindings.el (mode-line-collapse-minor-modes-to): New
option for the collapsed lighter of minor modes.
(mode-line-collapse-minor-modes): Mention that the collapsed
lighter can be customized.
(mode-line--minor-modes): Use the new option.

lisp/bindings.el

@@ -431,7 +431,8 @@ a menu, so this function is not useful for non-menu keymaps."
 
 (defcustom mode-line-collapse-minor-modes nil
   "Minor modes for which mode line lighters are hidden.
-Hidden lighters are collapsed into one.
+Hidden lighters are collapsed into one, which is customizable via option
+`mode-line-collapse-minor-modes-to'.
 
 The value could be a list (MODES ...) which means to collapse lighters
 only for MODES, or a list (not MODES ...) which means to collapse all
@@ -445,6 +446,15 @@ lighters hidden."
   :group 'mode-line
   :version "31.1")
 
+(defcustom mode-line-collapse-minor-modes-to
+  (if (char-displayable-p ?…) " …" " ...")
+  "Lighter for collapsed minor modes.
+This is effective only when `mode-line-collapse-minor-modes' is non-nil."
+  :type 'string
+  :initialize #'custom-initialize-delay
+  :group 'mode-line
+  :version "31.1")
+
 (defcustom mode-line-modes-delimiters '("(" . ")")
   "Strings placed around the modes displayed in the mode line.
 These elements are placed around `mode-name' and `mode-line-modes'."
@@ -550,7 +560,7 @@ mouse-3: Toggle minor modes"
                       :parent mode-line-minor-mode-keymap
                       "<mode-line> <down-mouse-1>" menu
                       "<mode-line> <mouse-2>" #'describe-mode)))
-              `(:propertize ,(if (char-displayable-p ?…) " …" " ...")
+              `(:propertize mode-line-collapse-minor-modes-to
                             mouse-face mode-line-highlight
                             help-echo "Hidden minor modes\n\
 mouse-1: Display hidden minor modes\n\