ESM build to support imports from .mjs files

[btakita]

Following the technique presented in https://www.sensedeep.com/blog/posts/2021/how-to-create-single-source-npm-module.html, a library can have both cjs & mjs outputs.

[sindresorhus]

I plan to move this package to ESM this year, but not just yet. I also don't plan to preserve the CJS part.