bug#65307: 29.1; Add variable-pitch-text-mode minor mode (or customization for variable-pitch-mode)

[Rudi Schlatte <rudi@constantly.at>]

Hello,

Emacs 29.1 added the face `variable-pitch-text`, which looks nicer when
combined with fixed-width text in a buffer.  This happens for example in
org-mode buffers that contain both text and code snippets.

The existing minor mode `variable-pitch-mode` uses the face
`variable-pitch`.  It would be nice to either have a
`variable-pitch-text-mode` minor mode, or a customization that lets the
user choose between the `variable-pitch` and `variable-pitch-text` faces
for `variable-pitch-mode`.

`variable-pitch-text-mode` can be trivially defined by copy-and-pasting
`variable-pitch-mode` in face-remap.el:

(defun variable-pitch-text-mode (&optional arg)
  "Variable-pitch-text default-face mode.
An interface to `buffer-face-mode' which uses the `variable-pitch-text' face.
Besides the choice of face, it is the same as `variable-pitch-mode'."
  (interactive (list (or current-prefix-arg 'toggle)))
  (buffer-face-mode-invoke 'variable-pitch-text (or arg t)
			   (called-interactively-p 'interactive)))