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

.babelrc

@@ -19,7 +19,6 @@
     "@babel/preset-react"
   ],
   "plugins": [
-    "@babel/plugin-transform-modules-commonjs",
     ["@babel/plugin-transform-runtime", {
       "corejs": "2"
     }],
@@ -30,7 +29,6 @@
     [
       "babel-plugin-module-resolver",
       {
-        "root": ".",
         "alias": {
           "root": ".",
           "components": "./src/core/components",