build(deps): bump posthtml-markdownit from 1.3.1 to 3.1.0

Bumps [posthtml-markdownit](https://github.com/posthtml/posthtml-markdownit) from 1.3.1 to 3.1.0.
- [Release notes](https://github.com/posthtml/posthtml-markdownit/releases)
- [Commits](posthtml/posthtml-markdownit@v1.3.1...v3.1.0)

---
updated-dependencies:
- dependency-name: posthtml-markdownit
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

package.json

@@ -65,7 +65,7 @@
     "posthtml-extend": "^0.6.0",
     "posthtml-extra-attributes": "^2.0.0",
     "posthtml-fetch": "^3.0.0",
-    "posthtml-markdownit": "^1.3.1",
+    "posthtml-markdownit": "^3.1.0",
     "posthtml-match-helper": "^1.0.3",
     "posthtml-mso": "^3.1.0",
     "posthtml-postcss-merge-longhand": "^3.1.0",