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.lock

@@ -433,7 +433,7 @@ GEM
     rack-cors (1.0.2)
     rack-protection (2.0.5)
       rack
-    rack-proxy (0.6.4)
+    rack-proxy (0.6.5)
       rack
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
@@ -640,7 +640,7 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff
-    webpacker (3.5.5)
+    webpacker (4.0.2)
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
       railties (>= 4.2)
@@ -772,7 +772,7 @@ DEPENDENCIES
   twitter-text (~> 1.14)
   tzinfo-data (~> 1.2018)
   webmock (~> 3.5)
-  webpacker (~> 3.5)
+  webpacker (~> 4.0)
   webpush
 
 RUBY VERSION