Finish JSON reporter & use it for more reliable CMake/CTest integration

[horenmar]

Catch2 currently provides XML reporter for tooling integration, but CMake
has never figured out XML. Instead, it can parse JSON (with a very weird,
very CMake, API), so we need a JSON reporter for CMake and CTest integration
(e.g. catch_discover_tests). People are also generally more comfortable
with JSON than with XML.

Catch2 already understands JSON escaping and can list tests in JSON, but
rest of the reporter is not done, neither is adapting catch_discover_tests
for JSON output.