Update chai requirement from ^3.5.0 to ^4.2.0
Update chai requirement from ^3.5.0 to ^4.2.0

Updates the requirements on [chai](https://github.com/chaijs/chai) to permit the latest version.
- [Release notes](https://github.com/chaijs/chai/releases)
- [Changelog](https://github.com/chaijs/chai/blob/master/History.md)
- [Commits](https://github.com/chaijs/chai/commits/4.2.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
hexojs/hexo-generator-sitemap