Add serializer support for code_block and headings
Add serializer support for code_block and headings

This adds serializer rules for code_blocks and headings from level 2 to
6. This also abstracts the logic used to render marks on inline text in
paragraph nodes into a general function that works on text within
headings, code_blocks, and list items.

iFixit/prosemirror-wikitext,iFixit/prosemirror-wikitext