lib: re-export promises for esm

fs.promises, dns.promises, (and soon readline.promises) should all be exposed nicely to esm. This does so.

cc @nodejs/modules @addaleax @cjihrig

Checklist

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