Improve 402 version of IsBuiltinCalendar

[FrankYFTang]

The "15.6.1.1 IsBuiltinCalendar ( id )" say the following
"
This definition supersedes the definition provided in 12.1.2.

1. Let builtinCalendars be a List of Unicode BCP 47 calendar identifiers identifying the calendars for which the implementation provides the functionality of the constructed Temporal.Calendar objects. The list must include "iso8601". The ordering is irrelevant.
   "
   The problem is the term "Unicode BCP 47 calendar identifiers" is not clearly defined here. I suggest we made a editoral changes as below:

https://tc39.es/proposal-temporal/#sup-temporal-isbuiltincalendar

"
This definition supersedes the definition provided in 12.1.2. The following algorithm refers to UTS 35's Unicode BCP 47 U Extension.

1. Let builtinCalendars be a List of Unicode BCP 47 calendar identifiers identifying the calendars for which the implementation provides the functionality of the constructed Temporal.Calendar objects. The list must include "iso8601". The ordering is irrelevant.
   "

[FrankYFTang]

or maybe it should juse use

Let builtinCalendars be AvailableCalendars ( ).

see https://tc39.es/proposal-intl-enumeration/#sec-availablecalendars
since proposal-intl-enumeration is already Stage 3

[ptomato]

I agree, using AvailableCalendars seems like the best way to do this. Thanks for pointing it out, as I wasn't aware it existed!

Otherwise I think this is a duplicate of #541, so I'll close this one.