Add a fast path for String.p.startsWith(str) when length of str is 1.

Bug: v8:8400
Change-Id: I65e657549902dc3ad064a213d815dd098ce6455f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1491872
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60091}