Build for ES

[Hey]

Problem

I am trying to use this package in an ES project but it looks like this package is currently only building for commonjs.

Fix

I have added the build-es to the build script and I added a prepack to ensure it's always built before we pack & publish. Would be great if you can merge and publish these changes.

[swarthy]

ES modules build is not ready yet. There are some more requirements to get this done https://www.typescriptlang.org/docs/handbook/esm-node.html