benchmark: refactor helper into a class

This is the basis to refactor the helper to use modern class
language features such as private fields.

It also refactors the exports to use module.exports. That way it's
immediately clear what parts are exported.

PR-URL: nodejs#31396
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>