feat: emulate rust's process api more closely

Yoink some docs as well

src/ExitStatus.php

@@ -4,15 +4,53 @@ declare(strict_types=1);
 
 namespace Nih\CommandBuilder;
 
+/**
+ * Describes the result of a process after it has terminated.
+ *
+ * This class is used to represent the exit status or other termination of a
+ * child process. Child processes are created via the {@see Command} class and
+ * their exit status is exposed through the status method, or the wait method of
+ * a Child process.
+ */
 final class ExitStatus
 {
     public function __construct(
-        public readonly int $code,
+        private readonly ?int $code,
+        private readonly ?int $signal = null,
+        private readonly ?int $stoppedSignal = null,
     ) {
     }
 
+    /**
+     * Was termination successful? Signal termination is not considered a
+     * success, and success is defined as a zero exit status.
+     */
     public function success(): bool
     {
         return $this->code === 0;
     }
+
+    /**
+     * The exit code of the process, if any.
+     */
+    public function code(): ?int
+    {
+        return $this->code > -1 ? $this->code : null;
+    }
+
+    /**
+     * If the process was terminated by a signal, returns that signal.
+     */
+    public function signal(): ?int
+    {
+        return $this->signal;
+    }
+
+    /**
+     * If the process was stopped by a signal, returns that signal.
+     */
+    public function stoppedSignal(): ?int
+    {
+        return $this->stoppedSignal;
+    }
 }