Switch to terser-webpack-plugin for JS minification

[renchap]

UglifyJS2 (also known as uglify-es, which supports ES6) is no longer maintained. It has been forked into Terser and webpack replaced uglifyjs-webpack-plugin with it for Webpack 5: webpack/webpack#8036

I suggest that Webpacker switches to Terser as well as the currently user Uglify version has many unfixed bugs (especially when using ES6 output).

[bjankord]

I'd encourage the webpacker team to switch over to terser as well. The react team has identified that uglify-es breaks the new React build (16.6) due to bugs in uglify-es (and that uglify-es seems deprecated). So you probably want to get off that package.

[gauravtiwari]

Please feel free to make a PR :)

[bjankord]

This is resolved with this PR: #1816
The PR has been merged to master.