esm: fixup process Proxy to handle toString correctly

This fixes the following --experimental-modules bug:

import process from 'process';
// returns [object Object] instead of [object process]
console.log(Object.prototype.toString.call(process));

//cc @devsnek

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines