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

src/standalone/index.js

@@ -4,7 +4,7 @@ import ConfigsPlugin from "corePlugins/configs"
 
 // the Standalone preset
 
-let preset = [
+export default [
   TopbarPlugin,
   ConfigsPlugin,
   () => {
@@ -13,5 +13,3 @@ let preset = [
     }
   }
 ]
-
-module.exports = preset