feat(decorateClientConfig): Upgrade to version 4

[michaeltaranto]

Upgrades the decorators to support webpack v4. We have made it this far because most apps are running sku which handles this for you.

The primary change for consumers is around the handling of styles emitted from the style guide compilation. The client decorator now has a cssOutputLoader option allowing you to configure the final loader in the css pipeline.

BREAKING CHANGE: Removed the client decorator option extractTextPlugin in favour of
cssOutputLoader. Also upgrading from webpack 3 to 4 so any plugins/loaders would need to be
supported by 4.

Migration Guide

Consumers will need to migrate their applications to support webpack v4, which will be project specific based on the webpack config.

Beyond the webpack upgrade, only consumers using the extractTextPlugin option in the client decorator need to migrate to the new cssOutputLoader option. The extract-text-webpack-plugin is not supported in webpack 4, but mini-css-extract-plugin is essentially a drop in replacement.

-const ExtractTextPlugin = require('extract-text-webpack-plugin');
+const MiniCssExtractPlugin = require('mini-css-extract-plugin');

-const extractCss = new ExtractTextPlugin({
+const extractCss = new MiniCssExtractPlugin({
  filename: 'style.css'
});

const webpackConfig = {
  ...
  plugins: [
    extractCss
  ]
};

module.exports = decorateClientConfig(webpackConfig, {
-  extractTextPlugin: extractCss
+  cssOutputLoader: MiniCssExtractPlugin.loader
});