build(deps): bump prosemirror-markdown from 1.9.3 to 1.9.4

Bumps [prosemirror-markdown](https://github.com/prosemirror/prosemirror-markdown) from 1.9.3 to 1.9.4.
- [Release notes](https://github.com/prosemirror/prosemirror-markdown/releases)
- [Changelog](https://github.com/ProseMirror/prosemirror-markdown/blob/master/CHANGELOG.md)
- [Commits](ProseMirror/prosemirror-markdown@1.9.3...1.9.4)

---
updated-dependencies:
- dependency-name: prosemirror-markdown
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]