Add return type for PHP 8.1

[tdomy]

In PHP 8.1, PHP internal classes were updated with tentative return types.
So I added return types to prevent the following types of warnings.

Deprecated: Return type of MStaack\LaravelPostgis\Geometries\GeometryCollection::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /work/laravel-postgis/src/Geometries/GeometryCollection.php on line 72

[mstaack]

Nice