xgettext: Scheme: Recognize datum-comments.

xgettext: Scheme: Recognize datum-comments.

Reported by Florent Angly <florent.angly@gmail.com>
at <https://savannah.gnu.org/bugs/?61882>.

* gettext-tools/src/x-scheme.c (datum_comment_nesting_depth): New variable.
(read_object): Omit the normal processing of list elements if
datum_comment_nesting_depth is non-zero. Recognize datum comments.
(extract_whole_file): Initialize datum_comment_nesting_depth.
* gettext-tools/tests/xgettext-scheme-1: Add a test case with a datum comment.
* NEWS: Mention the change.