[Prism] Reset server on kernel.terminate

Fixes:

```bash
In Process.php line 1292:

  [Exception]
  Cannot rewind a generator that was already run

Exception trace:
  at app/vendor/symfony/process/Process.php:1292
 IteratorIterator->rewind() at app/vendor/symfony/process/Process.php:1292
 Symfony\Component\Process\Process->getDescriptors() at app/vendor/symfony/process/Process.php:307
 Symfony\Component\Process\Process->start() at app/vendor/stenope/stenope/src/Highlighter/Prism.php:44
```

src/Highlighter/Prism.php

@@ -52,6 +52,7 @@ public function stop(): void
         }
 
         $this->server->stop();
+        $this->server = null;
         $this->input->close();
     }