Removes polyfills for `Object.assign` and `Set` which, as far as I can tell, are unused in the
codebase. Updates `webpack` from 3 to 4. Replaces `babel-preset-es2015` with `babel-preset-env`.
Reduces the minified bundle size (`feather.min.js`) by **9.3kB** (72.4kB to 63.1kB). Does not change
any functionality of the library.