build(deps): bump flatted from 3.1.1 to 3.4.2 in /themes/mainroad

Bumps [flatted](https://github.com/WebReflection/flatted) from 3.1.1 to 3.4.2.
- [Commits](WebReflection/flatted@v3.1.1...v3.4.2)

---
updated-dependencies:
- dependency-name: flatted
  dependency-version: 3.4.2
  dependency-type: indirect
...

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Author: dependabot[bot]