Port String.prototype.startsWith from a CPP builtin to a Torque builtin.

Spec: https://tc39.github.io/ecma262/#sec-string.prototype.startswith

Bug: v8:8400
Change-Id: I51aff0b3a4126c17ab4f89763019fd7e4ba665d9
Reviewed-on: https://chromium-review.googlesource.com/c/1361340
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59355}