Should be updated to conform to proper ESM package

[bouk]

Hi there, right now I can't use this package in a pure ESM fashion, you can follow this gist to conform to the proper ESM standards: https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c#how-can-i-make-my-typescript-project-output-esm