Upgrade to Webpacker 6.0.

package.json

@@ -5,16 +5,25 @@
     "@babel/plugin-transform-react-jsx": "^7.10.4",
     "@fortawesome/fontawesome-free": "^5.11.2",
     "@rails/ujs": "^6.0.2-1",
-    "@rails/webpacker": "^5.0.0",
+    "@rails/webpacker": "^6.0.0-beta.4",
+    "css-loader": "^5.0.1",
     "dropzone": "^5.5.1",
     "hammerjs": "^2.0.8",
     "jquery": "3.5.1",
     "jquery-hotkeys": "^0.2.2",
     "jquery-ui": "^1.12.1",
+    "mini-css-extract-plugin": "^1.3.4",
     "mobx": "^6.0",
     "mobx-react": "^7.0",
+    "postcss": "^8.2.4",
+    "postcss-flexbugs-fixes": "^5.0.2",
+    "postcss-import": "^14.0.0",
+    "postcss-loader": "^4.2.0",
+    "postcss-preset-env": "^6.7.0",
     "preact": "^10.4.6",
     "rails-erb-loader": "^5.5.0",
+    "sass": "^1.32.5",
+    "sass-loader": "^10.1.1",
     "spark-md5": "^3.0.0",
     "tippy.js": "^6.2.3",
     "typeface-anton": "^0.0.72",
@@ -26,24 +35,31 @@
     "typeface-rokkitt": "^1.0.0",
     "typeface-unifrakturmaguntia": "^0.0.72",
     "typopro-web": "^4.2.2",
-    "webpack-cli": "^3.3.0"
+    "webpack": "^5.11.0",
+    "webpack-cli": "^4.2.0"
   },
   "devDependencies": {
+    "@webpack-cli/serve": "^1.2.2",
     "babel-eslint": "^10.1.0",
     "eslint": "^7.0.0",
-    "eslint-loader": "^4.0.0",
     "eslint-plugin-babel": "^5.3.0",
     "eslint-plugin-ignore-erb": "^0.1.1",
+    "eslint-webpack-plugin": "^2.4.3",
     "stylelint": "^13.0.0",
     "stylelint-config-standard": "^20.0.0",
-    "webpack-dev-server": "^3.8.0"
+    "webpack-dev-server": "^3.11.2"
+  },
+  "scripts": {
+    "eslint-all": "yarn run eslint --plugin eslint-plugin-ignore-erb --ext .js,.js.erb app/javascript/src/javascripts",
+    "stylelint-all": "yarn run stylelint 'app/javascript/src/styles/**/*.scss'"
+  },
+  "babel": {
+    "presets": [
+      "./node_modules/@rails/webpacker/package/babel/preset.js"
+    ]
   },
   "browserslist": [
     "defaults",
     "since 2012"
-  ],
-  "scripts": {
-    "eslint-all": "yarn run eslint --plugin eslint-plugin-ignore-erb --ext .js,.js.erb app/javascript/src/javascripts",
-    "stylelint-all": "yarn run stylelint 'app/javascript/src/styles/**/*.scss'"
-  }
+  ]
 }