Bump webpacker from 4.0.7 to 4.2.0 (#12416)

* Bump webpacker from 4.0.7 to 4.2.0

Bumps [webpacker](https://github.com/rails/webpacker) from 4.0.7 to 4.2.0.
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rails/webpacker/compare/v4.0.7...v4.2.0

)

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

* Use NODE_ENV=tests instead of test, to work around async modules not having a chunk