Support ESM

[slominskir]

Is your feature request related to a problem? Please describe.
Support the JavaScript standard module system (ESM). Some JS frameworks focus support on ESM now and you've got to do legacy work-arounds to import CJS modules. For example SvelteKit (with Vite).

Deno support (#850) also becomes less of a lift.

Describe the solution you'd like
Pick one of:

1. add a build step that takes the current CJS source and distributes both it and an ESM version
2. convert the source to ESM and add a build step that takes the ESM source and distributes both it and a CJS version

Additional context
None.