Description:
------------
See https://secure.php.net/domnodelist - It's surprising that it isn't Countable, and it should be fairly easy to do.

It has a `length` property already, and is a type of list.

Code using `count($domNodeList)` would start emitting notices in php 7.2, and currently the default `count` is unintuitive.

In php 7.2.0alpha1, and 7.1, and any other version:

```
php > $x = new Domnodelist();
php > echo count($x);

Warning: count(): Parameter must be an array or an object that implements Countable in php shell code on line 1
1
```