PARTIAL: Update eras to match latest spec

[Manishearth]

I realized this is more complicated than I thought; the assumption about which calendars support eras is scattered throughout the code.

I don't think I'll have the time to finish this. However I do wish to update test262 here; the old era codes are causing failures in unrelated tests.

[ptomato]

It's probably better to implement this first in the reference code (that we use for developing test262) https://github.com/tc39/proposal-temporal/tree/main/polyfill and then it'll eventually be pulled in here.
However, I'm aware that the calendar code is a bit hard to grasp, since it is in the weird situation of reverse-engineering its info from Intl.DateTimeFormat. It's fine if you just want to update test262 and we can take care of the reference code when we pull in a new test262 commit.

[Manishearth]

Thanks!!