Expose a module for runtime detection of opt-in/opt-out.

At times, addons will need to know if specific optional features are enabled or not. With the current architecture it is only possible to detect at runtime via `window.EmberENV._SOME_FLAG_HERE`, but we should not encourage that type of coupling to `EmberENV`.

As I see it we have a few options:

* Provide a `@ember/optional-features` module that can be imported and checked for various opt-in's. This would allow consumers to use an import and conditionals in their code based on a given optional flag value.
* Expose a custom babel plugin that can be used when an addon includes `@ember/optional-features` as a direct dependency. This babel plugin would provide custom build time replacement of `@ember/optional-features` based conditionals, and would therefore allow addons to include support for multiple variations of features without bloating the total asset size.