build: add EnTT library

include/entt/resource/fwd.hpp

@@ -0,0 +1,19 @@
+#ifndef ENTT_RESOURCE_FWD_HPP
+#define ENTT_RESOURCE_FWD_HPP
+
+#include "../stl/memory.hpp"
+
+namespace entt {
+
+template<typename>
+struct resource_loader;
+
+template<typename Type, typename = resource_loader<Type>, typename = stl::allocator<Type>>
+class resource_cache;
+
+template<typename>
+class resource;
+
+} // namespace entt
+
+#endif