Improve support for entering quotation marks

Improve support for entering quotation marks

* lisp/international/iso-transl.el (iso-transl-char-map): Add entries
for "low" single and double quotation marks.
(iso-transl-language-alist): Add convenient support for the official
German quoting style to the German language entry. It uses "low" double
quotes on the left, and *left* double quotes on the right.  Add support
for single guillemets to the French language entry.
* lisp/leim/quail/latin-post.el ("latin-postfix"): Add entries for
single, double and "low" Unicode quotation marks.  Add entries for
single guillemets.
* lisp/leim/quail/latin-pre.el ("latin-prefix"): And here.

* etc/NEWS: Announce the changes.

(Bug#70984)