StringPrototypeCharAt, StringPrototypeCharCodeAt, StringPrototypeCodePointAt

Bug: v8:8996
Change-Id: I7faaec880801cfe244654a257dc98f67d4bc39ea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1617807Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Z Nguyen-Huu <duongn@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#61706}