Consider setting `production` mode for webpack config
https://github.com/skpm/skpm/blob/0050accbcc7a9f76d8d1454e3e0714c52f46194c/packages/builder/src/utils/webpackConfig.js#L126-L127

I don't know whether it was intentional or not , but setting [webpack's mode](https://webpack.js.org/concepts/mode/) to 'production' (when `NODE_ENV == 'production'`) will result in more efficient builds by default. 
