Add the static library Catch2WithMain

It should provide a shared impl for all targets that need to link
against Catch2's implementation. However, due to limitations of
C++ linking and Catch2's v2 implementation, this is only experimental
and might not work under some circumstances.

src/catch_with_main.cpp

@@ -0,0 +1,2 @@
+#define CATCH_CONFIG_MAIN
+#include <catch2/catch.hpp>