Update to ESM

[danvk]

I've pinned the open dependency to 8.x, since newer versions force ESM on me. https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c

Here's a PR where I did that for literate-ts last year: https://github.com/danvk/literate-ts/pull/64