Export real ESM

Following up from https://github.com/google/zx/issues/1205.

I'm not exactly sure what my issue was but it had something to do with a circular dependency tree.

ESM supports those.

I don't know why my local was working but not GH CI.

Anyways, I noticed that this package is not exporting real ESM but actually taking the CJS build and then putting ESM over that.

I would kindly ask that real ESM be used and that CJS consumers use recent version of node to import ESM. Alternatively, dual publish the package which you almost are already, however, don't mix the ESM and CJS builds.

Thanks for any consideration!