Return exitCode from FormatCommand.run()

Related to https://github.com/dart-lang/sdk/issues/44582
diff --git a/lib/src/cli/format_command.dart b/lib/src/cli/format_command.dart
index 4432f7b..5b14a7e 100644
--- a/lib/src/cli/format_command.dart
+++ b/lib/src/cli/format_command.dart
@@ -156,6 +156,8 @@
       options.summary.show();
     }
 
-    return 0;
+    // Return the exitCode explicitly for tools which embed dart_style
+    // and set their own exitCode.
+    return exitCode;
   }
 }