Add shiftTimezone() method to Chronos

[dereuromark]

Summary

Adds a shiftTimezone() method that changes the timezone while keeping the local time. This is different from setTimezone() which converts the time to the new timezone.

Difference from setTimezone()

Method	Behavior
setTimezone()	Converts to new timezone (same moment in time, different wall clock)
shiftTimezone()	Keeps wall clock time (different moment in time, same wall clock)

Example

$dt = Chronos::create(2024, 6, 15, 10, 0, 0, 0, 'America/New_York');

// setTimezone converts - same moment, different wall clock
$converted = $dt->setTimezone('America/Chicago');
$converted->hour; // 9 (Chicago is 1 hour behind NY)

// shiftTimezone keeps wall clock - different moment
$shifted = $dt->shiftTimezone('America/Chicago');
$shifted->hour; // 10 (same wall clock time)