feat(isArray): support Array subclasses in `angular.isArray()`

Closes #15533
Closes #15541

BREAKING CHANGE:

Previously, `angular.isArray()` was an alias for `Array.isArray()`.
Therefore, objects that prototypally inherit from `Array` where not
considered arrays. Now such objects are considered arrays too.

This change affects several other methods that use `angular.isArray()`
under the hood, such as `angular.copy()`, `angular.equals()`,
`angular.forEach()`, and `angular.merge()`.

This in turn affects how dirty checking treats objects that prototypally
inherit from `Array` (e.g. MobX observable arrays). AngularJS will now
be able to handle these objects better when copying or watching.