doc: add WHATWG file URLs in fs module

Update fs module documentation adding WHATWG file URLS support for
relevant fs functions/classes.

PR-URL: nodejs#12670
Fixes: nodejs#12341
Ref: nodejs#10739
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Timothy Gu <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>