xgettext: PHP: Support heredoc and nowdoc strings.

xgettext: PHP: Support heredoc and nowdoc strings.

Reported:
by Bodo Graumann <mail@bodograumann.de> at <https://savannah.gnu.org/bugs/?27740>,
at <https://savannah.gnu.org/bugs/?35944>,
by Cédric Anne <canne@teclib.com> at <https://savannah.gnu.org/bugs/?62158>.

* gettext-tools/src/x-php.c (TAB_WIDTH): New macro.
(process_heredoc): New function.
(phase4_get): Process the heredoc or nowdoc string, instead of blindly skipping
it.
* gettext-tools/tests/xgettext-php-1: Add tests of heredoc strings with embedded
expressions.
* gettext-tools/tests/xgettext-php-2: Expect spaces before a heredoc end label
to be eaten. Add tests of heredoc strings and nowdoc strings.
* NEWS: Mention the improvement.