Upgrade to Webpack 4

[awhitty]

I mentioned this in https://github.com/LaserWeb/LaserWeb4/pull/573, but just opening this issue to track it. There are some useful docs https://webpack.js.org/migrate/

In my opinion, webpack is one of the more valuable packages to keep up to date, but there is no immediately motivating factor to do this update sooner vs. later.