Bump mdast-util-to-string from 3.2.0 to 4.0.0

Bumps [mdast-util-to-string](https://github.com/syntax-tree/mdast-util-to-string) from 3.2.0 to 4.0.0.
- [Release notes](https://github.com/syntax-tree/mdast-util-to-string/releases)
- [Commits](syntax-tree/mdast-util-to-string@3.2.0...4.0.0)

---
updated-dependencies:
- dependency-name: mdast-util-to-string
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>