Use stderr for outputing warnings instead of stdout

This fixes phpstorm integration, which cannot currrently correctly detect behat version because of output that's mixed with the output in this extension.

src/Extension.php

@@ -23,6 +23,7 @@
 use Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition;
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\ConsoleOutputInterface;
 use Symfony\Component\Console\Output\OutputInterface;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Loader\PhpFileLoader;
@@ -186,6 +187,10 @@ public function process(ContainerBuilder $container): void
         /** @var OutputInterface $output */
         $output = $container->get(CliExtension::OUTPUT_ID);
 
+        if ($output instanceof ConsoleOutputInterface) {
+            $output = $output->getErrorOutput();
+        }
+
         $filterConfig = $container->getParameter('behat.code_coverage.config.filter');
         $branchPathConfig = $container->getParameter('behat.code_coverage.config.branchAndPathCoverage');
         $cacheDir = $container->getParameter('behat.code_coverage.config.cache');