src,lib: rename FSReqWrap to FSReqCallback

Given that FSReqPromise does not inherit from FSReqWrap, FSReqWrap
should be renamed FSReqCallback to better describe what it does.

First of a few upcoming `fs` refactorings :)

PR-URL: https://github.com/nodejs/node/pull/21971


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>