[bigint] Fastpath for small base-10 toString

This special-cases the most common situation to avoid call overhead.

Bug: chromium:1224299
Change-Id: Ic4bac2d5565f9b7a174eed845952b596adecbc8f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2997584


Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75502}