transient-prefix: Add experimental transient-switch-frame slot

This is experimental and guaranteed to be changed in not fully (or at
all) backward compatible ways.  Only use this at this time if you are
prepared to deal with that.

lisp/transient.el

@@ -648,6 +648,7 @@ If `transient-save-history' is nil, then do nothing."
    (man-page    :initarg :man-page    :initform nil)
    (transient-suffix     :initarg :transient-suffix     :initform nil)
    (transient-non-suffix :initarg :transient-non-suffix :initform nil)
+   (transient-switch-frame :initarg :transient-switch-frame)
    (refresh-suffixes     :initarg :refresh-suffixes     :initform nil)
    (incompatible         :initarg :incompatible         :initform nil)
    (suffix-description   :initarg :suffix-description)
@@ -1763,9 +1764,12 @@ of the corresponding object."
          (return (eq default t))
          (map (make-sparse-keymap)))
     (set-keymap-parent map transient-predicate-map)
-    (when (memq (transient--resolve-pre-command
-                 (oref transient--prefix transient-non-suffix))
-                '(nil transient--do-warn transient--do-noop))
+    (when (or (and (slot-boundp transient--prefix 'transient-switch-frame)
+                   (transient--resolve-pre-command
+                    (not (oref transient--prefix transient-switch-frame))))
+              (memq (transient--resolve-pre-command
+                     (oref transient--prefix transient-non-suffix))
+                    '(nil transient--do-warn transient--do-noop)))
       (define-key map [handle-switch-frame] #'transient--do-suspend))
     (dolist (obj transient--suffixes)
       (let* ((cmd (oref obj command))