Handle dates in year 99 rolling over into year 100 better

[diesieben07]

Fixes #1389

From #1389:

The problem here is not diff, but actually plus:
DateTime.fromISO('0099-12-31', {zone: "utc"}).plus({days: 61}).toISODate() produces 0100-03-01, not 0100-03-02. What is happening is that deep inside Luxon it calls Date.UTC with a computed date object (day, month, etc.). Date.UTC however treats years before 100 as 19XX. So year 99 turns into 1999. Luxon works around this by checking if the year is < 100 and if so, using setUTCFullYear afterwards. Here is the problem: year 2000 is a leap year, year 100 is not. So if you do a plus operation that goes from year 99 into year 100, the native JavaScript Date object thinks it is crossing from year 1999 into year 2000 - and year 2000 is a leap year. Luxon then goes in and fixes it back to year 100 - but the damage is done, the calculation was done with "this is a leap year" in mind.