Add `mdast`, `mdast-usage` as dev-dependencies
Add `mdast`, `mdast-usage` as dev-dependencies

yukkurisinai/mdast,chcokr/mdast,ulrikaugustsson/mdast,wooorm/remark,eush77/remark,ulrikaugustsson/mdast,yukkurisinai/mdast,eush77/remark,tanzania82/remarks,chcokr/mdast