Add support for node's fs FileHandle class

[humphd]

The promises API for fs includes a sort of object style fd called a FileHandle, which exposes a bunch of async methods. We should add this.

[humphd]

https://github.com/nodejs/node/blob/1e23e3ceb3217f2b0e076864fdbbe874a8603e2f/lib/internal/fs/promises.js#L48