Add support for non-CommonMark extensions in `mdast_util_to_markdown`

Seems like the JS version has tables supported as extension: https://github.com/syntax-tree/mdast-util-gfm-table


Right now when you generate an AST and serialize that this error is reached: https://github.com/search?q=repo%3Awooorm%2Fmarkdown-rs%20unexpected-node&type=code