Migrate ember-debug to @ember/debug and @ember/canary-features.

[rwjblue]

• Move Ember.assert, Ember.deprecate, Ember.warn, Ember.info, Ember.debug to @ember/debug.
• Move Ember.FEATURES, Ember.FEATURES.isEnabled to @ember/canary-features
• Remove "build time generation" of feature flags module.
• Remove features information from features.json and inline it into @ember/canary-features/index.ts. The build time replacement still occurs, but the TS file is now the "source of truth".
• Remove externs.d.ts, it was only used for tracking feature flags which now live in a TS file anyways.
• Remove "warn on feature flags when running non-canary" concept. This can later be re-added, but should be build time (not runtime) as a normal addon warning.
• Remove EMBER_GLIMMER_DETECT_BACKTRACKING_RERENDER feature flag (replace it with a simple if (DEBUG) {} guard).
• Remove final remnants of "debug features" vs "production features". Now all features are the same for debug vs prod.
• Fix issue with .gitignore (previously disallowed any folders named DEBUG).

[runspired]

@rwjblue you might be interested in this: https://emberjs.com/api/ember/3.1/functions/@ember%2Fapplication%2Fdeprecations/deprecate

Basically we've all been doing it wrong and deprecate is exported in two locations. The RFC was for the package in the docs, although I think the debug package makes way more sense.

[rwjblue]

Yep, I knew of that, but since it makes zero sense to me coming from @ember/application/deprecations and @ember/debug has special behaviors with ember-cli-babel (auto-stripping and whatnot) I kept these together.

Likely will have to make a reexport from @ember/application/deprecations to satisify the docs and eventually deprecate that location...