src: make `FileHandle` a (readonly) `StreamBase`

This enables accessing files using a more standard pattern.

Once some more refactoring has been performed on the other existing
`StreamBase` streams, this could also be used to implement `fs`
streams in a more standard manner.

PR-URL: https://github.com/nodejs/node/pull/18936


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>