Warn if FEATURES flagging is used in non-canary, debug build. Fixes #9302

Fixes #9302.

My approach of regexing `Ember.VERSION` seems brittle...but I'm not aware of any other way to know. Comments/suggestions welcome, especially if you can reduce the amount of code this takes.

Note that this PR adds `Ember.ENV.DEBUG` true/false so that warnings are only attempted in debug builds and also so this can be confirmed in testing. If there is a better, existing convention lmk.
If we use `Ember.ENV.FEATURES`, the `for (var key in FEATURES)` inside will be basically `for (var key in undefined)` for most people who don't define flags. Figured this might be 0.00000001 ms faster...