Allow alternative era for Chinese calendar

[anba]

Year 1 in the Chinese calendar corresponds to 2637 BCE in ICU4X.

See https://docs.rs/icu/latest/icu/calendar/chinese/struct.Chinese.html#year-and-era-codes.

[ptomato]

Similar to #4101, I'm hesitant to modify the tests to allow both results here. Either the ICU4C result is correct, or the ICU4X result is. That's something that ICU4X needs to sort out.

[anba]

This is different from #4101, because the different epoch year isn't a ICU4C vs. ICU4X difference, but a design decision from the Temporal polyfill. The Temporal polyfill formats dates using Intl.DateTimeFormat, which outputs in many cases the related Gregorian year, cf. new Intl.DateTimeFormat("en-u-ca-chinese").formatToParts(new Date).

Reading https://github.com/unicode-org/icu/blob/0357501948d2f0ab43c891f446e68f19b07b442d/icu4c/source/i18n/chnsecal.h#L52-L60, it seems like ICU4C and ICU4X actually agree on 2637 BCE as the initial epoch year.

And a similar modification was already allowed in #4079.

[ptomato]

OK, thanks, got it. If ICU4C and ICU4X both agree on the result, then my comment doesn't apply.