src: fix fs.write() externalized string handling

* Respect `encoding` argument when the string is externalized.

* Copy the string when the write request can outlive the externalized
  string.

This commit removes `StringBytes::GetExternalParts()` because it is
fundamentally broken.

Fixes: https://github.com/nodejs/node/issues/18146
PR-URL: https://github.com/nodejs/node/pull/18216


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>