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/interfaces/catch_interfaces_reporter.cpp

@@ -187,6 +187,5 @@ namespace Catch {
     }
 
     IReporterFactory::~IReporterFactory() = default;
-    IReporterRegistry::~IReporterRegistry() = default;
 
 } // end namespace Catch