Fixes PHP-notice

Without having int as return type, we will generate notices like

```
Return type of Spatie\LaravelData\DataCollection::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /Users/ahoi/test/vendor/spatie/laravel-data/src/DataCollection.php on line 104
```

src/DataCollection.php

@@ -101,7 +101,7 @@ public function getIterator(): ArrayIterator
         return new ArrayIterator($this->transform(TransformationType::withoutValueTransforming()));
     }
 
-    public function count()
+    public function count(): int
     {
         return count($this->items);
     }