src: clean up argument assertions in node_file.cc

- Cache `args.Length()`
- Use `value.As<T>()` to cast the arguments

PR-URL: https://github.com/nodejs/node/pull/18192


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>