This CL adds a fast path to String#startsWith(s) if s is a
single character string.

Bug: v8:8400
Change-Id: Ibd6a9d1e46d98f41c198d2b579208e25003eedb0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1525362Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61504}