refactor!: clean up stdio interface

with an interface no less!

src/ExitStatus.php

@@ -12,12 +12,12 @@ namespace Nih\CommandBuilder;
  * their exit status is exposed through the status method, or the wait method of
  * a Child process.
  */
-final class ExitStatus
+final readonly class ExitStatus
 {
     public function __construct(
-        private readonly ?int $code,
-        private readonly ?int $signal = null,
-        private readonly ?int $stoppedSignal = null,
+        private ?int $code,
+        private ?int $signal = null,
+        private ?int $stoppedSignal = null,
     ) {
     }