Split IReporterRegistry into its own header file

Doing this removes `<map>` from the include set of the base reporter
interface, and thus from bunch more TUs. This provides about 1.5%
improvements in the debug build of the static library, and 1% in
release build.

src/catch2/internal/catch_commandline.cpp

@@ -11,6 +11,7 @@
 #include <catch2/catch_config.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
 #include <catch2/interfaces/catch_interfaces_registry_hub.hpp>
+#include <catch2/interfaces/catch_interfaces_reporter_registry.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter.hpp>
 
 #include <fstream>