ICU's Islamic calendar implementations always set ERA=0, even for dates
before the Hijra (622 CE), using negative year values instead. However,
the CLDR defines two eras: "Anno Hegirae" (era 0) and "Before Hijrah"
(era 1). ECMA-402 expects distinct era names in formatToParts output.
Similarly, ICU's Coptic calendar has an empty CLDR era 0 name, causing
the era parts to be omitted entirely from formatted output.
This patch adds another icu::Calendar subclass to handle these cases.
