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/SelfTest/UsageTests/Exception.tests.cpp

@@ -4,7 +4,7 @@
  */
 
 #include <catch2/catch_test_macros.hpp>
-#include <catch2/catch_matchers_string.h>
+#include <catch2/matchers/catch_matchers_string.h>
 
 #include <string>
 #include <stdexcept>