Fix crash on \addlyrics in MusicXML export

[wattdave]

Problem

ly musicxml crashes when a \score contains music followed by \addlyrics:

\version "2.26.0"
\score {
  \new Staff { \relative c' { d1 c2 c | bes bes } }
  \addlyrics { Du tout plon -- giet }
}

AttributeError: 'list' object has no attribute 'pickup'
  File "ly/musicxml/xml_objs.py", line 113, in iterate_bar

(0.9.7 fails the same way with 'list' object has no attribute 'obj_list'.)

Root cause

LyricMode in ly/musicxml/lymus2musxml.py only switched to lyric mode for
\addlyrics; unlike \lyricsto, it never set up a LyricsSection. So
Mediator.new_lyrics_text() appended each [text, syllabic, nr] list straight
into self.insert_into.barlist, which at that point is the ScorePart. The
part's barlist then contained plain lists next to Bar objects, and
IterateXmlObjs.iterate_bar() crashed on them.

Fix

• LyricMode: for \addlyrics, call a new Mediator.new_addlyrics_section(),
  which collects the lyrics in a LyricsSection that remembers the section it
  follows.
• End: for \addlyrics, call a new Mediator.check_addlyrics(), which closes
  the last syllable, restores insert_into and merges the lyrics into the
  preceding music with the existing ScoreSection.merge_lyrics(). This mirrors
  how \lyricsto is already handled.

This fixes the crash, and the lyrics now also appear in the exported MusicXML as
<lyric> elements. Before, they were never exported.

Tests

• Added tests/test_xml_files/addlyrics.{ly,xml} and test_addlyrics in
  tests/test_xml.py. The fixture has seven notes and seven syllables, with
  single, begin and end syllables.
• Before the fix, test_addlyrics fails with the AttributeError. After the fix,
  the full suite gives 19 passed and 2 xfailed. The 2 xfails were already there.

Not addressed

• \new Lyrics \lyricsto "v" {...} still produces no <lyric> elements in
  some layouts. Example: lyricsto-no-lyrics.ly.
• Files that combine \new Voice = "x" << \var >> \addlyrics { \lyricvar } with
  variables fail with 'NoneType' object has no attribute 'name', and print
  'ParseSource' object has no attribute 'LayoutContext' and 'Keyword'
  warnings. Minimal example: voice-variable-addlyrics-crash.ly (the crash is fixed in Create an implicit part for a Voice outside a Staff #197). Real-world source: Brumel, Du tout plongiet, CPDL #19603 (Brumel-DuTout.ly, LilyPond 2.10).

These are separate issues and are left for follow-ups.

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