Esbuild doesn't minify assets by default

[murny]

One little change from moving from webpack to esbuild, is the fact that esbuild doesn't minify assets by default?

It's not a huge issue, but asset sizes have gone up a little as a result in production:

• application.css went from ~270KB to ~335KB
• application.js went from ~800KB to ~1.6 MB
• admin.js went from ~95KB to ~7KB (this is more to do with webpack and how they are rebundling all the libraries again in the separate packs. ESbuild doesn't do this, hence the massive savings.)

This should be a simple fix (think it just needs a --minify flag), but requires some investigation on the best way to do this.

More info: https://github.com/rails/jsbundling-rails/wiki#different-builds-for-production-and-developmenttest-environments