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-e2e.babel.js

@@ -0,0 +1,18 @@
+/**
+ * @prettier
+ */
+
+// The standard dev config doesn't allow overriding contentBase via the CLI,
+// which we do in the npm scripts for e2e tests. 
+//
+// This variant avoids contentBase in the config, so the CLI values take hold.
+
+import devConfig from "./dev.babel"
+
+// set the common e2e port 3230
+devConfig.devServer.port = 3230
+
+// unset contentBase
+delete devConfig.devServer.contentBase
+
+export default devConfig