cmstd: Modernize CMake system headers

Provide a standardized way to handle the C++ "standard" headers
customized to be used with current CMake C++ standard constraints.
Offer under directory `cm` headers which can be used as direct
replacements of the standard ones.  For example:

    #include <cm/string_view>

can be used safely for CMake development in place of the `<string_view>`
standard header.

Fixes: #19491

Source/cmFileCommand.cxx

@@ -20,7 +20,9 @@
 #include <utility>
 #include <vector>
 
-#include "cm_memory.hxx"
+#include <cm/memory>
+
+#include "cm_sys_stat.h"
 
 #include "cmAlgorithms.h"
 #include "cmArgumentParser.h"
@@ -44,7 +46,6 @@
 #include "cmSubcommandTable.h"
 #include "cmSystemTools.h"
 #include "cmTimestamp.h"
-#include "cm_sys_stat.h"
 #include "cmake.h"
 
 #if !defined(CMAKE_BOOTSTRAP)