s&box: JSON serialization with Sandbox.Json — handles engine types that System.Text.Json misses

In s&box, JSON serialization uses Sandbox.Json, not System.Text.Json directly. The Sandbox.Json helper handles s&box-specific types (Resources, GameObjects, Colors, Vectors) that System.Text.Json doesn't know about. Using System.Text.Json directly on s&box types produces incorrect or missing output.