LibCore+LibTimeZone: Support time zone names in Core::DateTime::parse

LibCore currently cannot depend on LibTimeZone directly. All build-time
code generators depend on LibCore, so there'd be a circular dependency:
LibCore -> LibTimeZone -> GenerateTZData -> LibCore.

So to support parsing time zone names and applying their offsets, add a
couple of weakly-defined helper functions. These work similar to the way
AK::String declares some methods that LibUnicode defines. Any user who
wants to parse time zone names (from outside of LibCore itself) can link
against LibTimeZone to receive full support.

Userland/Libraries/LibTimeZone/CMakeLists.txt

@@ -2,6 +2,7 @@ include(${SerenityOS_SOURCE_DIR}/Meta/CMake/time_zone_data.cmake)
 
 set(SOURCES
     ${TIME_ZONE_DATA_SOURCES}
+    DateTime.cpp
     TimeZone.cpp
 )
 set(GENERATED_SOURCES ${CURRENT_LIB_GENERATED})