Support ESM

[Josh-Cena]

This module can't be used in ESM because it only exports a default property but default-import in Node would import the entire module.exports.

I see several solutions here:

1. As what Docusaurus did, add a module.exports = combinePromises. Seems hacky, but works.
2. Add a named export combinePromises.
3. Publish as dual-package.
4. In the next major, migrate to ESM.