Issue #4692 - Rewrite web request task to avoid engine self-reference

Issue #4692 - Rewrite web request task to avoid engine self-reference

Rewrite the C++ implementation of JS "_webRequest.GET" to use v8 types instead
of the wrapped ones that include shared pointers to the engine.

Change initialization of global JS property "_webRequest", moving it to the
constructor of `JsEngineInternal`. Initialize the associated JS function with
`MakeCallback`, which uses a plain engine pointer and not a smart pointer.

Add value functions that avoid shared pointers to the engine, one for string
conversion and three for setting JS object properties. These substitute for
equivalent methods in `JsValue`.

Keep a single `JsEnginePtr` in the web request task object. This compensates
for the deficiency that we have no other way of keeping an engine in existence
for the duration of the task.