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/Compilation.tests.cpp

@@ -29,7 +29,7 @@ std::ostream& operator<<(std::ostream& out, foo::helper_1403 const&) {
 ///////////////////////////////
 
 #include <catch2/catch_test_macros.hpp>
-#include <catch2/catch_matchers_string.h>
+#include <catch2/matchers/catch_matchers_string.h>
 
 #include <cstring>