Upgrade to webpack 4.0 alpha (with javascript/esm mode)

[dtinth]

The changelog of each webpack 4 alpha, and notable parts relevant to this project:

• alpha.0 webpack 4.0.0-alpha.0 feedback webpack/webpack#6064

  • With --mode, our webpack config can be simplified.
  • Module type javascript/esm allows more strict bundling (e.g. checks for nonexistent imports), and more predictable behavior when importing non-ESM modules.

• alpha.1 webpack 4.0.0-alpha.1 feedback webpack/webpack#6132

• alpha.2 webpack 4.0.0-alpha.2 feedback webpack/webpack#6179

  • Faster uglification

[resir014]

Webpack 4 beta is now released, and if I have the time, I could work on it this week. 🚀