Move reporter_registrars.hpp to reporters/

examples/210-Evt-EventListeners.cpp

src/CMakeLists.txt

src/catch2/catch_all.hpp

src/catch2/internal/catch_reporter_registry.cpp

src/catch2/reporters/catch_reporter_registrars.hpp

src/catch2/reporters/catch_reporters_all.hpp

tests/ExtraTests/X21-PartialTestCaseEvents.cpp

tests/ExtraTests/X22-BenchmarksInCumulativeReporter.cpp

tests/ExtraTests/X23-CasingInReporterNames.cpp

tests/SelfTest/TestRegistrations.cpp