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/_helpers.js

@@ -0,0 +1,17 @@
+/**
+ * @prettier
+ */
+
+import { gitDescribeSync } from "git-describe"
+
+export function getRepoInfo() {
+  try {
+    return gitDescribeSync(__dirname)
+  } catch (e) {
+    console.error(e)
+    return {
+      hash: "noGit",
+      dirty: false,
+    }
+  }
+}