build: add entt library

include/entt/core/concepts.hpp

@@ -0,0 +1,17 @@
+#ifndef ENTT_CORE_CONCEPTS_HPP
+#define ENTT_CORE_CONCEPTS_HPP
+
+#include "../stl/type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Specifies that a type is not a cv-qualified reference.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept cvref_unqualified = stl::is_same_v<stl::remove_cvref_t<Type>, Type>;
+
+} // namespace entt
+
+#endif