Add standardized copyright notice + SPDX identifier to source files

This should also be done for test files, but that has lower priority.

src/catch2/internal/catch_assertion_handler.cpp

@@ -1,3 +1,10 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
 #include <catch2/internal/catch_assertion_handler.hpp>
 #include <catch2/interfaces/catch_interfaces_config.hpp>
 #include <catch2/internal/catch_context.hpp>