Split some node serializers for rich text editor in separate files.
Moves them from markdown_serializer.js and serialization_helper.js
to their own files under a serializer directory. The
markdown_serializer.js and serialization_helper.js files were getting
too large.

The nodes moved include description_item, description_list,
details_content, details, emoji, figure_caption, figure,
footnote_definition, footnote_reference, frontmatter, heading,
horizontal_rule, list_item, loading.