Add lisp-data-mode for editing non-code Lisp data

Add lisp-data-mode for editing non-code Lisp data

Fixes: bug#40573
The new mode can be used stand-alone or inherited from by modes
intended to edit programs. The existing emacs-lisp-mode and lisp-mode
are examples.

Thanks to Juri Linkov and Basil L. Contovounesios for researching some
data files in Emacs that can be automatically set to use the new mode.

* lisp/files.el (auto-mode-alist): Add entry for ".dir-locals" and
".dir-locals-2"

* lisp/emacs-lisp/lisp-mode.el: (lisp-data-mode): New major mode.
(lisp-mode): Inherit from lisp-data-mode.  Set special lisp-mode
stuff here.

* lisp/progmodes/elisp-mode.el (emacs-lisp-mode): Inherit from
lisp-data-mode.

* lisp/bookmark.el (bookmark-insert-file-format-version-stamp):
Use lisp-data-mode.

* lisp/saveplace.el (save-place-alist-to-file): Use
lisp-data-mode.

* lisp/net/eww.el (eww-write-bookmarks): Use lisp-data-mode.

* lisp/net/nsm.el (nsm-write-settings): Use lisp-data-mode.

* lisp/net/tramp-cache.el (tramp-dump-connection-properties): Use
lisp-data-mode.

* etc/NEWS: Mention lisp-data-mode.

* doc/lispref/modes.texi (Example Major Modes): Update example.