Improve detection of std::uncaught_exceptions support

The problem was that Catch2 did not reliably include `<exception>`
before it checked for the feature test macro for
`std::uncaught_exceptions`. To avoid overhead of including
`<exception>` everywhere, the configuration check was split out
into a separate header.

Closes #2021

projects/CMakeLists.txt

@@ -124,6 +124,7 @@ set(INTERNAL_HEADERS
         ${HEADER_DIR}/internal/catch_common.h
         ${HEADER_DIR}/internal/catch_compiler_capabilities.h
         ${HEADER_DIR}/internal/catch_config.hpp
+        ${HEADER_DIR}/internal/catch_config_uncaught_exceptions.hpp
         ${HEADER_DIR}/internal/catch_console_colour.h
         ${HEADER_DIR}/internal/catch_context.h
         ${HEADER_DIR}/internal/catch_debug_console.h