Move matcher implementation to their own subfolder

In the future we can expect many more matchers, so let's give them
a place to live.

Also moved matcher-related internal files to `internal` subfolder.
Ideally we should sort out all of our source code, but that will
have to come later.

tests/ExtraTests/X01-PrefixedMacros.cpp

@@ -5,8 +5,8 @@
 // This won't provide full coverage, but it might be worth checking
 // the other branch as well
 #include <catch2/catch_test_macros.hpp>
-#include <catch2/catch_matchers_generic.hpp>
-#include <catch2/catch_matchers_string.h>
+#include <catch2/matchers/catch_matchers_generic.hpp>
+#include <catch2/matchers/catch_matchers_string.h>
 
 #include <type_traits>
 #include <stdexcept>