build(deps): bump posthtml-postcss-merge-longhand from 3.1.2 to 3.1.3

Bumps [posthtml-postcss-merge-longhand](https://github.com/posthtml/posthtml-postcss-merge-longhand) from 3.1.2 to 3.1.3.
- [Release notes](https://github.com/posthtml/posthtml-postcss-merge-longhand/releases)
- [Commits](posthtml/posthtml-postcss-merge-longhand@v3.1.2...v3.1.3)

---
updated-dependencies:
- dependency-name: posthtml-postcss-merge-longhand
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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