Add a new function on the public API to allow serializing a function to
a string using the built-in toString() implementation, allowing
serialization without worrying about untrusted author script overriding
the toString() implementation. This is similar in nature to
Object::ObjectProtoToString() (but that only returns "[object Function]"
for any passed function).

Add tests for the same.

Bug: chromium:1144841
Change-Id: Ie4c29b870034c0817c23bf91f9424f956098823d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2514768Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70976}