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
Fixes: https://github.com/nodejs/node/issues/22728
Backport-PR-URL: https://github.com/nodejs/node/pull/22731
PR-URL: https://github.com/nodejs/node/pull/18216



Reviewed-By: James M Snell <jasnell@gmail.com>