fs: move method definition from header

We are defining the definition of function in the header file itself. It
should be defined in the source file instead of the header file.

PR-URL: https://github.com/nodejs/node/pull/36256


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>