Allow transient-switches to have descriptions for choices

[haji-ali]

This small patch allows choices in transient-switches to be each a cons so that options that accept non-descriptive values can still be displayed with descriptive text.

I created this as a pull-request draft since, if this is of interest, a similar change to transient-option would also be needed for consistency.

Example usage:

(defun snowcone-eater (args)
  (interactive (list (transient-args 'transient-toys-snowcone-eater)))
  (message "%S" args))

(transient-define-argument transient-toys--snowcone-flavor ()
  :description "Flavor of snowcone"
  :class 'transient-switches
  :key "s"
  :argument-format "--snowcone=%s"
  :argument-regexp "\\(--snowcone=\\(2\\|3\\|4\\|8\\)\\)"
  :choices '("none" 
             (2 . "grape")
             (3 . "orange")
             (4 . "cherry")
             (9 . "lime")))

(transient-define-prefix transient-toys-snowcone-eater ()
  "Eat a flavored snowcone!"
  :value '("--orange-snowcone")

  ["Arguments"
   (transient-toys--snowcone-flavor)
   ("C-c C-s" "Show options"
   snowcone-eater
   :transient t)])

(transient-toys-snowcone-eater)