Bump memdown from 5.1.0 to 6.0.0

Bumps [memdown](https://github.com/Level/memdown) from 5.1.0 to 6.0.0.
- [Release notes](https://github.com/Level/memdown/releases)
- [Changelog](https://github.com/Level/memdown/blob/master/CHANGELOG.md)
- [Commits](Level/memdown@v5.1.0...v6.0.0)

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

package.json

@@ -32,7 +32,7 @@
     "hallmark": "^3.1.0",
     "level-community": "^3.0.0",
     "level-compose": "^1.0.0",
-    "memdown": "^5.0.0",
+    "memdown": "^6.0.0",
     "nyc": "^15.1.0",
     "protocol-buffers": "^4.0.2",
     "standard": "^16.0.3",