Better builds

- https://levelup.gitconnected.com/code-splitting-for-libraries-bundling-for-npm-with-rollup-1-0-2522c7437697
- https://medium.com/@kelin2025/so-you-wanna-use-es6-modules-714f48b3a953
- https://medium.com/passpill-project/files-with-mjs-extension-for-javascript-modules-ced195d7c84a