Integrate CommonsChunkPlugin for Webpack into extension-webpack

Webpack has a built in plugin to extract common libraries like vendor libraries in to a separate bundle. (https://webpack.js.org/plugins/commons-chunk-plugin/)

Make this easily configurable for Elixir.

Something like:

This should add the webpack configuration listed here: https://webpack.js.org/plugins/commons-chunk-plugin/#explicit-vendor-chunk