feat(clock): support chrono Time Zone extensions.

Use chrono Calendars and Time Zones (P0355R7, P1466R3) when available
instead of the `date` library.
Verified with a patched build of a recent GCC 13 snapshot.

src/modules/clock.cpp

@@ -9,7 +9,6 @@
 #include <sstream>
 #include <type_traits>
 
-#include "util/date.hpp"
 #include "util/ustring_clen.hpp"
 #ifdef HAVE_LANGINFO_1STDAY
 #include <langinfo.h>