src: clean up node_file.h

- Move inline functions into an `-inl.h` file
- Move override function definitions into `.cc` files
- Remove `using` statements from header files
- Make data fields of classes private
- Mark classes at the end of hierarchies as `final`

This is also partially being done in an attempt to avoid
a particular internal compiler error, see
https://github.com/nodejs/node/pull/30475#issuecomment-554740850
for details.

PR-URL: https://github.com/nodejs/node/pull/30530


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>