Separate string adapter from storage policy

src/ArduinoJson/Strings/IsString.hpp

@@ -0,0 +1,17 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright Benoit Blanchon 2014-2021
+// MIT License
+
+#pragma once
+
+#include <ArduinoJson/Polyfills/type_traits.hpp>
+
+namespace ARDUINOJSON_NAMESPACE {
+
+template <typename T, typename Enable = void>
+struct IsString : false_type {};
+
+template <typename TChar>
+struct IsString<const TChar*> : IsString<TChar*> {};
+
+}  // namespace ARDUINOJSON_NAMESPACE