fs: add `bufferSize` option to `fs.opendir()`

Add an option that controls the size of the internal
buffer.

Fixes: https://github.com/nodejs/node/issues/29941

PR-URL: https://github.com/nodejs/node/pull/30114


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>