Supporting ESM & CJS in one package

[tiagosiebler]

https://antfu.me/posts/publish-esm-and-cjs

[tiagosiebler]

Implemented in another module successfully: https://github.com/tiagosiebler/accountstate

Since it's a somewhat significant change, I'm going to hold off implementing it here unless more people ask for it. If you'd like to see an ESM build, please let me know.