fix: mini-css-extract-plugin does not return the class in 2.5.0

See webpack-contrib/mini-css-extract-plugin#896

Author: jgraichen

webpack.config.js

@@ -1,7 +1,7 @@
 //
 const { resolve, join } = require("path");
 
-const MiniCssExtractPlugin = require("mini-css-extract-plugin");
+const MiniCssExtractPlugin = require("mini-css-extract-plugin").default;
 const postcssEnv = require("postcss-preset-env");
 const cssnano = require("cssnano");