Move interactive `transpose-regions' to Lisp

Move interactive `transpose-regions' to Lisp

Define `transpose-regions' in Lisp, because its complex interactive
spec was ungainly in C, and change the C version to non-interactive
`transpose-regions-internal'.  The Lisp function is just a wrapper
around the C function, which still does all the work.

* lisp/simple.el (transpose-regions): New wrapper function, with
  interactive spec taken from old C `transpose-regions'.

* src/editfns.c (Ftranspose_regions): Rename to...
  (Ftranspose_regions_internal): ...here, and remove interactive spec.

Discussion on Emacs Devel:

  From: Karl Fogel
  To: Emacs Development
  Cc: Richard Copley, Charles A. Roelli
  Subject: Re: [Emacs-diffs] master b88e7c8: \
           Make transpose-regions interactive (Bug#30343)
  Date: Fri, 16 Mar 2018 10:23:31 -0500
  Message-ID: <87po44jb7w.fsf@red-bean.com>

  https://lists.gnu.org/archive/html/emacs-devel/2018-03/msg00555.html