ixml parser: skip a DOCTYPE and a processing instruction

[oblomov-dev]

What is wrong

Neither has a branch of its own, so both reach the branch for a start or close
tag, where the tag regex does not match them and the parse dies in
ASSERT ls_match-offset = 0 — an assertion, so no CATCH cx_root absorbs it:

parse( |<!DOCTYPE note SYSTEM "note.dtd"><root><item>A</item></root>| ).
" ASSERTION_FAILED

parse( |<?xml-stylesheet type="text/xsl" href="a.xsl"?><root><item>A</item></root>| ).
" ASSERTION_FAILED

A document that names its DTD, or that carries a stylesheet instruction ahead
of the root element, cannot be read at all — and both stand where they stand
in plenty of files that are otherwise ordinary XML.

The change

The processing instruction needs no new code. The xml declaration was
already skipped by a branch that matches '<?xml *' and then looks for ?>
— and ?> is what every processing instruction ends with, so the pattern
becomes '<?*' and the same three lines serve both:

IF lv_xml CP '<?*'.
  FIND FIRST OCCURRENCE OF '?>' IN lv_xml MATCH OFFSET lv_offset.
  ASSERT lv_offset > 0.
  lv_offset = lv_offset + 2.

The DOCTYPE gets a branch of its own. It ends at its >, except when it
carries an internal subset — there the > of the subset is not the end of the
declaration, and ]> is:

ELSEIF lv_xml CP '<!DOCTYPE*'.
  FIND FIRST OCCURRENCE OF '>' IN lv_xml MATCH OFFSET lv_offset.
  ASSERT sy-subrc = 0.
  FIND FIRST OCCURRENCE OF '[' IN lv_xml MATCH OFFSET lv_subset.
  IF sy-subrc = 0 AND lv_subset < lv_offset.
    FIND FIRST OCCURRENCE OF ']>' IN lv_xml MATCH OFFSET lv_offset.
    ...

Neither carries content of the document, so neither becomes a node — the same
shape as the comment in #1230.

Tests

• parse_doctype — <!DOCTYPE note SYSTEM "note.dtd"> before the root
• parse_doctype_subset — <!DOCTYPE note [<!ELEMENT note (#PCDATA)>]>, where
  the > inside the subset must not end the declaration
• parse_processing_instruction — a stylesheet instruction before the root
• parse_instruction_in_element — <?target data?> between two elements

npm test is green: abaplint: 0 issue(s) found, 735 file(s) analyzed, and
the full unit run passes.