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.

include/modules/clock.hpp

@@ -1,8 +1,7 @@
 #pragma once
 
-#include <date/tz.h>
-
 #include "ALabel.hpp"
+#include "util/date.hpp"
 #include "util/sleeper_thread.hpp"
 
 namespace waybar::modules {