Add Array.prototype.filter polyfill.

This is currently required (and only used by) the composable-computed-properties feature. If that feature is enabled it will not work in non-ES5 browsers (specifically it is not available in IE8 and lower).

This was the root cause of the issue reported in #4211 as the feature was accidentally enabled in the beta branch.

`Ember.EnumerableUtils.filter` is only used in [packages/ember-metal/lib/computed.js#L589](https://github.com/emberjs/ember.js/blob/master/packages/ember-metal/lib/computed.js#L589) (it is used by the `selectDependentCPs` function).

The original `composable-computed-properties` PR included this under a separate feature flag (`array-filter`) in f34661cb86, but that was not added to the `features.json` or `FEATURES.md` so it was removed in 71a21bd0 (as it was assumed to be dead code).

Since this is only used by an already feature-flagged feature and it seems to be the polyfill directly from MDN, I didn't think we would need to separately feature flag this. If y'all disagree I'll be happy to flag it.

/cc @hjdivad