Add AO: DefaultMergeFields
Spec Text:
https://tc39.es/proposal-temporal/#sec-temporal.calendar.prototype.mergefields
https://tc39.es/proposal-temporal/#sec-temporal-defaultmergefields


Bug: v8:11544
Change-Id: I270f8bffb79e57ef50736ae7ce87cfa53f9cafb1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3388428Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79795}