Bump webpacker from 3.5.5 to 4.0.2 (#10277)

Bumps [webpacker](https://github.com/rails/webpacker) from 3.5.5 to 4.0.2.
- [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/v3.5.5...v4.0.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>
Co-authored-by: Yamagishi Kazutoshi <ykzts@desire.sh>

Gemfile

@@ -86,7 +86,7 @@ gem 'tty-command', '~> 0.8', require: false
 gem 'tty-prompt', '~> 0.18', require: false
 gem 'twitter-text', '~> 1.14'
 gem 'tzinfo-data', '~> 1.2018'
-gem 'webpacker', '~> 3.5'
+gem 'webpacker', '~> 4.0'
 gem 'webpush'
 
 gem 'json-ld', '~> 3.0'