Add compat adapter for ember-get-config

[alexlafroscia]

I've been struggling to get ember-get-config to work with the Vite packager due to the fact that it's desired behavior is to re-export a module from the host app.

At @ef4's suggestion I totally replaced the original way of doing this with a new approach that is Embroider-compatible. We can look up the host app's config at build-time and serialize it into the module's index.js; this maintains the add-on's API without having the export from the host app.

One drawback is that the config/environment contents are now serialized twice in the payload, but I don't think that's necessarily a huge deal.

One other approach I had tried was using the setOwnConfig/getOwnConfig API from the macros package but

1. I couldn't get it working; I overrode the options getter and added the @embroider/macros property but getOwnConfig introduced into the module's index.js would return undefined
2. It ultimately was just extra steps; the approach taken here is simpler by cutting out that whole step

[alexlafroscia]

The CI failure seems like a browser timeout in both jobs, but I can't re-run CI myself

[alexlafroscia]

Rebased from master in hopes of getting a passing build this time around

[ef4]

Thanks for working on this.