Spec Text:
https://tc39.es/proposal-temporal/#sec-temporal.plaintime.prototype.add
https://tc39.es/proposal-temporal/#sec-temporal.plaintime.prototype.subtract

Bug: v8:11544
Change-Id: I0d684eec9eebfd26eceb6bb4c360025a33bd4c3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3617390Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80957}