Add support for IS NOT operator

Allows for doing

```php
$query->where(['field IS NOT' => null]);
```
is it worth consolidating these two expressions?

    if ($operator === 'is not') {
        if ($value === null) {
            return new UnaryExpression('IS NOT NULL', $expression, UnaryExpression::POSTFIX);
        }

        $operator = '!=';
    }

Same for `is`.