common: Move system time zone string detection

Moves it from Settings to Common::TimeZone, since this algorithm doesn't
depend on the setting. It also lets us use it in other libraries.

common: Various fixes

time_zone: Don't double up the std::abs

Too many absolute values were causing mirrored time zones to resolve
as the same.

src/common/time_zone.h

@@ -3,15 +3,21 @@
 
 #pragma once
 
+#include <array>
 #include <chrono>
 #include <string>
 
 namespace Common::TimeZone {
 
+[[nodiscard]] const std::array<const char*, 46>& GetTimeZoneStrings();
+
 /// Gets the default timezone, i.e. "GMT"
 [[nodiscard]] std::string GetDefaultTimeZone();
 
 /// Gets the offset of the current timezone (from the default), in seconds
 [[nodiscard]] std::chrono::seconds GetCurrentOffsetSeconds();
 
+/// Searches time zone offsets for the closest offset to the system time zone
+[[nodiscard]] std::string FindSystemTimeZone();
+
 } // namespace Common::TimeZone