transient-prefix: Add mode-line-format slot

:initform is set to 'undefined so that passing :mode-line-format nil
to transient-define-prefix overrides transient-mode-line-format.

docs/transient.org

@@ -563,6 +563,9 @@ Also see [[* Common Suffix Commands]].
   ~transient-key-exit~ (if allowed and they exit the transient) is
   used to draw the line.
 
+  This user option may be overridden if ~:mode-line-format~ is passed
+  when creating a new prefix with ~transient-define-prefix~.
+
   Otherwise this can be any mode-line format.  See [[info:elisp#Mode
   Line Format]], for details.
 
@@ -1917,6 +1920,9 @@ functions use ~describe-function~.
 - ~display-action~ determines how this prefix is displayed, overriding
   ~transient-display-buffer-action~.  It should have the same type.
 
+- ~mode-line-format~ is this prefix's mode line format, overriding
+  ~transient-mode-line-format~.  It should have the same type.
+
 - ~incompatible~ A list of lists.  Each sub-list specifies a set of
   mutually exclusive arguments.  Enabling one of these arguments
   causes the others to be disabled.  An argument may appear in

lisp/transient.el

@@ -682,6 +682,7 @@ If `transient-save-history' is nil, then do nothing."
    (transient-non-suffix :initarg :transient-non-suffix :initform nil)
    (transient-switch-frame :initarg :transient-switch-frame)
    (display-action :initarg :display-action :initform nil)
+   (mode-line-format :initarg :mode-line-format :initform 'undefined)
    (refresh-suffixes     :initarg :refresh-suffixes     :initform nil)
    (environment          :initarg :environment          :initform nil)
    (incompatible         :initarg :incompatible         :initform nil)
@@ -3839,10 +3840,10 @@ have a history of their own.")
         (setq tab-line-format nil))
       (setq header-line-format nil)
       (setq mode-line-format
-            (if (or (natnump transient-mode-line-format)
-                    (eq transient-mode-line-format 'line))
-                nil
-              transient-mode-line-format))
+            (let ((format (transient--mode-line-format)))
+              (if (or (natnump format) (eq format 'line))
+                  nil
+                format)))
       (setq mode-line-buffer-identification
             (symbol-name (oref transient--prefix command)))
       (if transient-enable-popup-navigation
@@ -3886,11 +3887,16 @@ have a history of their own.")
         (fit-window-to-buffer window nil (window-height window))
       (fit-window-to-buffer window nil 1))))
 
+(defun transient--mode-line-format ()
+  (let ((format (oref transient--prefix mode-line-format))
+        (default (oref-default transient--prefix mode-line-format)))
+    (if (eq format default) transient-mode-line-format format)))
+
 (defun transient--separator-line ()
-  (and-let* ((height (cond ((not window-system) nil)
-                           ((natnump transient-mode-line-format)
-                            transient-mode-line-format)
-                           ((eq transient-mode-line-format 'line) 1)))
+  (and-let* ((format (transient--mode-line-format))
+             (height (cond ((not window-system) nil)
+                           ((natnump format) format)
+                           ((eq format 'line) 1)))
              (face `(,@(and (>= emacs-major-version 27) '(:extend t))
                      :background
                      ,(or (face-foreground (transient--key-face nil 'non-suffix)