housekeeping: upgrade to webpack@4 (via #5454)

* build new core webpack config

* fix exports in Webpack; use ESM syntax throughout

* add bundle config

* add standalone config

* add style config

* prettier...

* add dev config

* delete legacy webpack scripts

* rewire npm scripts to use new webpack configs

* cache babel-loader results

* fix e2e dev servers

* update core Webpack modules

* update loaders to latest

* remove unused loaders

* update Webpack plugins

* add mode flags to Webpack configs

* remove plugin invocations that are now production-standard in v4

* update webpack-cli

* add webpack perf size limit flags

* replace ExtractText with MiniCssExtract + IgnoreAssets

* UglifyJsPlugin -> TerserPlugin

* fix PostCSS processing

* enable Terser sourcemaps

* webpack/style -> webpack/stylesheets

webpack/dev.babel.js

@@ -0,0 +1,66 @@
+/**
+ * @prettier
+ */
+
+import path from "path"
+import { HotModuleReplacementPlugin } from "webpack"
+import configBuilder from "./_config-builder"
+import styleConfig from "./stylesheets.babel"
+
+const devConfig = configBuilder(
+  {
+    minimize: false,
+    mangle: false,
+    sourcemaps: true,
+    includeDependencies: true,
+  },
+  {
+    mode: "development",
+    entry: {
+      "swagger-ui-bundle": [
+        "./src/polyfills.js", // TODO: remove?
+        "./src/core/index.js",
+      ],
+      "swagger-ui-standalone-preset": [
+        "./src/polyfills", // TODO: remove?
+        "./src/standalone/index.js",
+      ],
+      "swagger-ui": "./src/style/main.scss",
+    },
+
+    performance: {
+      hints: false
+    },
+
+    output: {
+      library: "[name]",
+      filename: "[name].js",
+      chunkFilename: "[id].js",
+    },
+
+    devServer: {
+      port: 3200,
+      publicPath: "/",
+      disableHostCheck: true, // for development within VMs
+      stats: {
+        colors: true,
+      },
+      hot: true,
+      contentBase: path.join(__dirname, "../", "dev-helpers"),
+      host: "0.0.0.0",
+    },
+
+    plugins: [new HotModuleReplacementPlugin()],
+  }
+)
+
+// mix in the style config's plugins and loader rules
+
+devConfig.plugins = [...devConfig.plugins, ...styleConfig.plugins]
+
+devConfig.module.rules = [
+  ...devConfig.module.rules,
+  ...styleConfig.module.rules,
+]
+
+export default devConfig