fix: handle stdio properly

examples/stdio.php

@@ -0,0 +1,35 @@
+<?php
+
+use Nih\CommandBuilder\Command;
+use Nih\CommandBuilder\Stdio;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+$child = (new Command('cat'))
+    ->stdin(Stdio::piped())
+    ->stdout(Stdio::piped())
+    ->spawn();
+
+$child->stdin?->write('Hello, World!');
+$output = $child->waitWithOutput();
+
+var_dump($output);
+
+// object(Nih\CommandBuilder\Output)#6 (3) {
+//   ["stdout"]=>
+//   string(13) "Hello, World!"
+//   ["stderr"]=>
+//   NULL
+//   ["code"]=>
+//   object(Nih\CommandBuilder\ExitStatus)#4 (1) {
+//     ["code"]=>
+//     int(0)
+//   }
+// }
+
+(new Command('echo'))
+    ->arg('Hello, World!')
+    ->stdout(Stdio::inherit())
+    ->status();
+
+// Hello, World!