fix(support): use key() instead of current() in valid()

Bug reproduction:
```
$arr = new ImmutableArray([true, false, true]);
foreach ($arr as $value) {
    // Will only iterate once, stopping at the `false` value
}
```

Author: xHeaven

packages/support/src/Arr/IsIterable.php

@@ -24,7 +24,7 @@ public function key(): string|int|null
 
     public function valid(): bool
     {
-        return $this->current() !== false;
+        return $this->key() !== null;
     }
 
     public function rewind(): void