More spec ports, and small updates to GetTemporalUnit typings.

[12wrigja]

Like other spec port PR's, I will merge this as-is without squashing to retain history.

[12wrigja]

FYI that I'll be on vacation till the 6th of September (2022), so I won't respond till then. There's no rush on this review.

I've asked some colleagues to take a look at some of the more complex type-system implementation I used for PrepareTemporalFields to see if there's any way to simplify it further whilst retaining the useful properties the current impl has:

• preparing fields returns an object with all the given field names as required properties with the types from the "potential owners" of that field.
  • all the other properties of the potential owners that weren't specificed in the field name list are marked as optional - this allows them to be assigned to without casting / complain from tsc. This isn't perfect - it does mean in some cases casts are still required to change the properties type after assignment (e.g. ecmascript.ts L4574 is a good example).

[12wrigja]

@justingrant @ptomato I think Steve and I are done with back-and-forth on this, and I'd appreciate if you could take a look. Thanks!