Get rid of JsValuePtr and use JsValue instead

[sergz]

Reasons

On practice it's visible that in most or even in all cases we don't need JsValuePtr, merely JsValue is enough, and the code can be still efficient because currently used compilers already support move-semantics. The code without addition wrapper (std::shared_ptr<JsValue>) should be simpler to use and less error-prone.

What to change

Gradually get rid of JsValuePtr.

[sergz]

I think next steps should be in commmits respectively

• JsValue::NewCallback and JsEngine::SetProperty
• JsEngine::NewValue

afterwards

• JsEngine::NewObject

[abpbot]

Some commits referencing this issue have landed:

• ​Issue 5034 - Part 1: Pass a JsValue directly to SetProperty() and return from GetProperty()
• ​Issue 5034 - Part 2: Have NewCallback() return a plain JsValue

[abpbot]

Some commits referencing this issue have landed:

• ​Issue 5034 - Part 3: Create plain JsValue instead of JsValuePtr
• ​Issue 5034 - Part 4: JsEngine::Evaluate() return a JsValue