AvailableCurrencies and default fallback for unsupported currencies in NumberFormat

[anba]

https://tc39.es/proposal-intl-enumeration/#sec-availablecurrencies:

[...] currencies for which the implementation provides the functionality of Intl.DisplayNames and Intl.NumberFormat objects.

But in PartitionNumberPattern, step 8.k.iii:

[...] If the implementation does not have such a representation of currency, use currency itself.

So strictly speaking an implementation is required to provide functionality for any currency code, even if the currency code doesn't match any currency. I don't know how to best describe that situation in AvailableCurrencies().

[sffc]

TG2 discussion: https://github.com/tc39/ecma402/blob/master/meetings/notes-2022-10-06.md#intl-enumeration-api-for-stage-4

Conclusion: Try to clarify the wording. This does not need to block Stage 4.