fs: refactor FSReqWrap and After

Separate FSReqWrap definition into a new node_file.h.

Add Reject and Resolve methods to encapsulate the callbacks and
make the constructor, destructor protected instead of private
in preparation to make FSReqWrap subclassable for the Promises
implementation.

Rework and simplify the After function slightly in preparation
for a refactor.

Introduce the node::fs namespace instead of using an anonymous
namespace for fs methods.

PR-URL: https://github.com/nodejs/node/pull/17689


Reviewed-By: Anna Henningsen <anna@addaleax.net>