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/core.babel.js

@@ -0,0 +1,28 @@
+/**
+ * @prettier
+ */
+
+import configBuilder from "./_config-builder"
+
+const result = configBuilder(
+  {
+    minimize: true,
+    mangle: true,
+    sourcemaps: true,
+    includeDependencies: false,
+  },
+  {
+    entry: {
+      "swagger-ui": [
+        "./src/polyfills.js", // TODO: remove?
+        "./src/core/index.js",
+      ],
+    },
+
+    output: {
+      library: "SwaggerUICore",
+    },
+  }
+)
+
+export default result