Split MessageInfo into its own header

This is first step towards splitting apart reporter implementation
and `catch_string.hpp`.

src/catch2/interfaces/catch_interfaces_reporter.cpp

@@ -7,6 +7,7 @@
 
 #include <catch2/interfaces/catch_interfaces_reporter.hpp>
 #include <catch2/internal/catch_console_colour.hpp>
+#include <catch2/catch_message.hpp>
 #include <catch2/internal/catch_list.hpp>
 #include <catch2/internal/catch_text.hpp>
 #include <catch2/internal/catch_string_manip.hpp>

src/catch2/interfaces/catch_interfaces_reporter.hpp

@@ -14,7 +14,7 @@
 #include <catch2/catch_totals.hpp>
 #include <catch2/catch_test_case_info.hpp>
 #include <catch2/catch_assertion_result.hpp>
-#include <catch2/catch_message.hpp>
+#include <catch2/internal/catch_message_info.hpp>
 #include <catch2/internal/catch_option.hpp>
 #include <catch2/internal/catch_stringref.hpp>