1. Fix formatToParts show "plusSign" for -0 by using signbit instead of < 0
2. Fix crash bug of formatToParts while notation is either "engineering" or
   "scientific".

Bug: v8:9513
Change-Id: I36b0b995c157dc642b9c201694d0ac0bb0396988
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1710148
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62856}