Note that the AST now carries the name “mdast”, meaning
utilities (such as mdast-util-inject) keep their name.
This name-change includes the `mdast` formatter, which also changed
to `remark`.
Additionally:
* Update relating packages.
* Fix a bug where `text` nodes without value were
[generated](6b905aefb1/lib/output/markdown_ast.js (L144)).