Add unimplementation macro for String case till the resolution of
https://github.com/tc39/proposal-intl-numberformat-v3/pull/82

Bug: v8:10776
Change-Id: Ic59d532bb78a9f56a0c121e71e0f9e585bb8c9d7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497619Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79358}