Start of epoch for Islamic observational calendar doesn't match CLDR/ICU4C about icu4x HOT 1 OPEN

@roozbehp, do you have thoughts on this?

Reingold says on page 106:

The calendar is computed, by the majority of the Muslim world, starting at sunset of Thursday, July 15, 622 c. e. (Julian), the year of Mohammed’ s migration to Medina from Mecca. 2 The introduction of the calendar is often attributed to the Caliph ‘Umar in 639 c. e., but there is evidence that it was in use before his succession. In essence, Muslims count r. d. 227015 = Friday, July 16, 622 c. e. (Julian) as the beginning of the Islamic year 1, that is, as Muḥarram 1, a. h. 3 1, and thus we define islamic-epoch def () = fixed-from-julian 622 c. e. july 16 (7.1)

from icu4x.