Support an ESM-based config file

[simonihmig]

I tried to see if we can support the config file proposed in this experimental addon to be ESM-based (i.e. config/addons.mjs). However I failed to do so.

AFAIU, the constraints here are:

• our "addon-main" module (index.js) has to be CJS
• you cannot require() an ESM module from CJS
• you can do so using import(), but that makes it async
• as it seems, you cannot use that when this would need to happen synchronously during the included hook (that's also when @embroider/macros does all its Ember-CLI plumbing work). When done asynchronously (trying that here), the setConfig() calls seems to happen too late in time to have an effect

So without introducing support for async included in EmberCLI itself, I wouldn't know how to make this work with the constraints we have here!? 🤔

Also, when looking at upstreaming this feature to @embroider/macros, I wouldn't know if that makes things any easier, as again the blocker seems that we have to do the setup work synchronously within the included hook...