Switch from uglifyjs-webpack-plugin to terser-webpack-plugin

[sfusato]

• uglifyjs-webpack-plugin uses uglify-es, which is no longer maintained.

uglify-es has been forked and the new project is called terser. For projects that need ES6 support, use it instead of uglify-es, since all new development will be happening over there (and things like webpack are switching over to it shortly too):
https://github.com/fabiosantoscode/terser

• uglify-js does not have support for ES6, and the existing ES6 support (uglify-es) has been deprecated
• Webpack 5 will use Terser as well as default minimizer.
• seems Webpack 4 switched to terser as well because they consider this a non-breaking change
  webpack/webpack#8350 (comment)

[Gazler]

Will need to double check that this works as expected. Thanks for bringing it to our attention!

[robinvdvleuten]

I use this setup as well in my Phoenix project and can confirm that this gives the same minified results compared to Uglify. I did a deep dive into this already for Next.js (vercel/next.js#5021), where they switched to Terser as well after I suggested it.

[Gazler]

Thanks <3