how to properly expose an ES class function residing inside a module?
I'm not aware of any best practice here, so this answer is my own hunch.

I would stick with using an uppercase 'C' for the exported member name.

In JavaScript, the usual protocol is to use an uppercase name for a function that should be new from, rather than called directly e.g.

function Config()

to show that the function should be used with a new call, to treat it as a constructor. e.g.

var config = new Config();

As you are exposing a class from your module, in other words a function that can be new'd up from, then I would stick with using a capital C.