Add function that returns daylight saving time offset

[cjoecker]

It would be great to have a function that returns the daylight saving time offset for a specific date and timezone.

I propose this implementation:

export function getDaylightSavingTimeOffset(date: Date, timezone: string) {
	const today = fromZonedTime(date, timezone);
	const todayOffset = getTimezoneOffset(timezone, today);
	const januaryOffset = getTimezoneOffset(
		timezone,
		new Date(today.getFullYear(), 0, 1),
	);
	const julyOffset = getTimezoneOffset(
		timezone,
		new Date(today.getFullYear(), 6, 1),
	);
	const stdTimezoneOffset = Math.min(januaryOffset, julyOffset);
	return (todayOffset - stdTimezoneOffset);
}