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
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kyle