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.
:+1: