Split markdown serializer for some nodes in separate files (iteration 2)
What does this MR do and why?
Split markdown serializer for some nodes in separate files
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.
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Screenshots
No visual change. Run spec/frontend/content_editor/services/markdown_serializer_spec.js
to confirm that serialization still works as expected.