Fix for syntactic whitespace regressions before templated list elts

* echo "* {{bullet}}" where bullet emits a new list item would
  serialize to "* \n{{bullet}}".  While this is just a syntactic diff,
  this might still be considered annoying.

* This patch fixes the regression by finetuning and cleaning up
  newline separator constraints for list items.

* Added a new parser test to capture this test case.  This test
  fails in the html2wt mode because an extra newline is emitted
  at the start of output.  I've marked another parser test with
  this same wacky behavior.  This indicates a missing constraint
  for lists.  I tried adding it in, but it breaks other kinds of
  HTML, so this one is trickier and since it is anyway somewhat
  independent of what I am fixing here, I am leaving it for another
  patch at another time with some testing notes.

* Fixes both of these:
  - echo '* {{búho}}' | node parse --prefix es --wt2wt
  - http://localhost:8000/_rt/es/Guillermo_I_de_Hesse-Kassel

Change-Id: I8525aa63abeb06fd72126d3ae9ecb0a1cb17c4c9
Merge "Fix for syntactic whitespace regressions before templated list elts"