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/IntrospectiveTests/CmdLine.tests.cpp

@@ -5,7 +5,7 @@
 
 #include <catch2/catch_approx.h>
 #include <catch2/catch_test_macros.hpp>
-#include <catch2/catch_matchers_string.h>
+#include <catch2/matchers/catch_matchers_string.h>
 #include <catch2/catch_test_spec_parser.h>
 #include <catch2/catch_test_case_info.h>
 #include <catch2/catch_config.hpp>

tests/SelfTest/IntrospectiveTests/StringManip.tests.cpp

@@ -1,5 +1,5 @@
 #include <catch2/catch_test_macros.hpp>
-#include <catch2/catch_matchers_vector.h>
+#include <catch2/matchers/catch_matchers_vector.h>
 #include <catch2/catch_string_manip.h>
 
 static const char * const no_whitespace = "There is no extra whitespace here";

tests/SelfTest/IntrospectiveTests/Tag.tests.cpp

@@ -3,8 +3,8 @@
  *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 
-#include <catch2/catch_matchers_string.h>
-#include <catch2/catch_matchers_vector.h>
+#include <catch2/matchers/catch_matchers_string.h>
+#include <catch2/matchers/catch_matchers_vector.h>
 #include <catch2/catch_tag_alias_registry.h>
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/catch_test_case_info.h>

tests/SelfTest/IntrospectiveTests/ToString.tests.cpp

@@ -1,5 +1,5 @@
 #include <catch2/catch_enum_values_registry.h>
-#include <catch2/catch_matchers_vector.h>
+#include <catch2/matchers/catch_matchers_vector.h>
 #include <catch2/catch_test_macros.hpp>
 
 enum class EnumClass3 { Value1, Value2, Value3, Value4 };