Export ES6 module via package.json

[dhoko]

👋 cf https://github.com/rollup/rollup/wiki/pkg.module

We need to use this key inside the package.json to be able to use it inside another project.
ex: eslint will throw an error: 50:32 error Unable to resolve path to module 'web-streams-polyfill' import/no-unresolved`.

You also need to publish the file index.es6.js via npm.