This is a reland of c7c5b492

Changes since revert:
- Save position instead of using PushBack
- Allow private-name-surrogate-pair to fail on no_i18n builds

Original change's description:
> [scanner] Combine surrogate pairs at start when scanning private names
>
> Bug: v8:12523
> Change-Id: Ic3779fe6f20965d177d99d0a570a735df72e4fde
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3366994
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Shu-yu Guo <syg@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#78493}

Bug: v8:12523
Change-Id: I8a92953549f5b38bfa004488db42bf9d835e1222
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3368361Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78513}