Move CATCH_INTERNAL_STRINGIFY macro into its own header

src/catch2/internal/catch_preprocessor_internal_stringify.hpp

@@ -0,0 +1,19 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+#ifndef CATCH_PREPROCESSOR_INTERNAL_STRINGIFY_HPP_INCLUDED
+#define CATCH_PREPROCESSOR_INTERNAL_STRINGIFY_HPP_INCLUDED
+
+#include <catch2/catch_user_config.hpp>
+
+#if !defined(CATCH_CONFIG_DISABLE_STRINGIFICATION)
+  #define CATCH_INTERNAL_STRINGIFY(...) #__VA_ARGS__##_catch_sr
+#else
+  #define CATCH_INTERNAL_STRINGIFY(...) "Disabled by CATCH_CONFIG_DISABLE_STRINGIFICATION"_catch_sr
+#endif
+
+#endif // CATCH_PREPROCESSOR_INTERNAL_STRINGIFY_HPP_INCLUDED