xgettext: Add E4X support to JavaScript scanner

xgettext: Add E4X support to JavaScript scanner

Reported by Piotr Drąg at: <https://savannah.gnu.org/bugs/?40125>.
* src/xgettext.h (enum lexical_context_ty): New enumeration items
lc_xml_open_tag, lc_xml_close_tag, lc_xml_content.
* src/x-javascript.c (phase5_scan_xml_markup): New
function.
(phase5_get): Handle '<', '>', '/', '=', '{', and '}' specially
to support E4X.
(enum token_type_ty): New enumeration item token_type_equal.
(xml_element_depth): New variable.
(inside_embedded_in_xml): New variable.
(extract_javascript): Initialize those variables.
* tests/Makefile.am (TESTS): Add xgettext-javascript-6.
* tests/xgettext-javascript-6: New file.