[ActiveSupport core_ext] Allow to use `attr_internal` with `private` modifier

This was available as attr_protected but not anymore.

If the return value of attr_internal was the list of created methods, one could write

class Foo
  private *(attr_internal :bar, :baz)

Even if it’s not for that case, knowing what methods were added by attr_internal would be very nice.

The changes to achieve that can be seen at Allow proper privatization of internal attributes by rewritten · Pull Request #1 · rewritten/rails · GitHub