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>

jest.config.js

@@ -1,29 +0,0 @@
-module.exports = {
-  projects: [
-    '<rootDir>/app/javascript/mastodon',
-  ],
-  testPathIgnorePatterns: [
-    '<rootDir>/node_modules/',
-    '<rootDir>/vendor/',
-    '<rootDir>/config/',
-    '<rootDir>/log/',
-    '<rootDir>/public/',
-    '<rootDir>/tmp/',
-  ],
-  setupFiles: [
-    'raf/polyfill',
-  ],
-  setupTestFrameworkScriptFile: '<rootDir>/app/javascript/mastodon/test_setup.js',
-  collectCoverageFrom: [
-    'app/javascript/mastodon/**/*.js',
-    '!app/javascript/mastodon/features/emoji/emoji_compressed.js',
-    '!app/javascript/mastodon/locales/locale-data/*.js',
-    '!app/javascript/mastodon/service_worker/entry.js',
-    '!app/javascript/mastodon/test_setup.js',
-  ],
-  coverageDirectory: '<rootDir>/coverage',
-  moduleDirectories: [
-    '<rootDir>/node_modules',
-    '<rootDir>/app/javascript',
-  ],
-};