(optim): use HardSourceWebpackPlugin to cache to disk across builds

https://github.com/mzgoddard/hard-source-webpack-plugin/

This should be added if initial compilation times begin to slow down. Since development builds are done with incremental compilation and in-memory (with `webpack-serve`), initial compilation would be the most affected by this. Unsure if I'd want to use a cache at all for production (fresh build is preferable) and it might not be useful for production anyway as the configuration is very different.

Some things to consider:
1. This is planned to be implemented into core webpack in v5: https://github.com/webpack/webpack/issues/6527 . When webpack 5 might land is an unknown.
2. This will need some custom configuration at the very least for `confighash` in order to include the Babel, PostCSS, and Browserslist configs (etc)