1. Added method `debug::GetBigIntStringValue`.
2. Used the method in BigInt BiDi serialization.

Bug: v8:13043
Change-Id: I6047d2ea7657e8bb891f5099971deed49bd3e31b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3749185
Auto-Submit: Maksim Sadym <sadym@chromium.org>
Commit-Queue: Maksim Sadym <sadym@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81609}