Unify IReporterRegistry and ReporterRegistry

To keep the compilation firewall effect, the implementations
are hidden behind a PIMPL. In this case it is probably not
worth it, but we can inline it later if needed.

tests/SelfTest/IntrospectiveTests/Reporters.tests.cpp

@@ -12,7 +12,6 @@
 #include <catch2/catch_config.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter_factory.hpp>
-#include <catch2/interfaces/catch_interfaces_reporter_registry.hpp>
 #include <catch2/internal/catch_console_colour.hpp>
 #include <catch2/internal/catch_enforce.hpp>
 #include <catch2/internal/catch_list.hpp>