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

[rwjblue]

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.

[rwjblue]

@chancancode - Thoughts?

[rwjblue]

Oh, also, IMHO we should do both options...

[bendemboski]

This should probably also expose whether the user explicitly opted in/out (as opposed to simply accepting the default), so it can be used to address emberjs/ember.js#16849.

[rwjblue]

Ya, agreed, though I think that it may need to be a separate API. I think most of the time you just want to know enabled or disabled, but in a somewhat rare set of cases you care if the user explicitly chose the setting or not...

[Gaurav0]

As there has been no movement on this, Ember Twiddle will be using EmberENV as an intimate api.