Make ESM module

[aslakhellesoy]

I started working on this (declared type module in package.json and tsconfig.json). But I reverted to commonjs because there are other issues to fix first, like #6.

[aurelien-reeves]

In progress: #17

[davidjgoss]

Unlike with most of our other JS packages, I don't think we need to make this a hybrid module. The consumers are browsers rather than Node.js programs, and all major bundlers and test runners now support ESM, so we should be able to just switch straight to ESM.