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

Gemfile.lock

@@ -660,7 +660,7 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff (>= 0.4.0, < 2.0.0)
-    webpacker (4.0.7)
+    webpacker (4.2.0)
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
       railties (>= 4.2)
@@ -800,7 +800,7 @@ DEPENDENCIES
   twitter-text (~> 1.14)
   tzinfo-data (~> 1.2019)
   webmock (~> 3.7)
-  webpacker (~> 4.0)
+  webpacker (~> 4.2)
   webpush
 
 RUBY VERSION