Spec Text:
https://tc39.es/proposal-temporal/#sec-temporal.zoneddatetime.prototype.startofday

Bug: v8:11544
Change-Id: I475e03fa9ba43290896a906524414cfbddd1f7bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3385610
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80754}