breaking: main module is commonjs, reference esm module via package.json

[andreineculau]

• Fixes: #18
• Breaking change: [x]

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went with a breaking solution, since there are a few other PRs that are about to break... might just group them all

UPDATE Refs:

• http://2ality.com/2017/04/setting-up-multi-platform-packages.html
• https://babeljs.io/blog/2018/06/26/on-consuming-and-publishing-es2015+-packages
• https://github.com/rollup/rollup/wiki/pkg.module