Handle und month codes

[Manishearth]

We hit debug assertions here.

@sffc Should ICU4X be producing these at all? I think perhaps we should only ever produce valid month codes, producing M00/M13 when out of range.

[sffc]

Once we adopt Pinqi then this should be a moot point

[Manishearth]

@sffc This was happening in Hijri

const v2 = Temporal.PlainMonthDay;
const v7 = {
    calendar: "islamic-umalqura",
    day: 18,
    era: "bh",
    eraYear: 65536,
    monthCode: "M12",
};
v2.from(v7);

[Manishearth]

Yes, this will be a moot point, but we should perhaps do something less unexpected for 2.1.