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.

src/catch2/catch_assertionhandler.cpp

@@ -14,9 +14,9 @@
 #include <catch2/catch_enforce.h>
 #include <catch2/catch_debugger.h>
 #include <catch2/catch_interfaces_registry_hub.h>
-#include <catch2/catch_capture_matchers.h>
+#include <catch2/internal/catch_capture_matchers.h>
 #include <catch2/catch_run_context.h>
-#include <catch2/catch_matchers_string.h>
+#include <catch2/matchers/catch_matchers_string.h>
 
 namespace Catch {