Add a normal method to convert PropertyLValue into Value

[neko-para]

Currently, a PropertyLValue can only be converted into Value via the type convertion.

// cast into var
Napi::Value val = obj["key"];
val.ToString();

// explicit cast
Napi::Value(obj["key"]).ToString();

It‘s fairly common to directly use it as a value, but the methods above are quite annoying. The first one needs to name the value, and the second one needs to move the cursor back. Thus I'd suggest adding a helper function behaves like the converter.

demo usage

obj["key"].value().ToString();

possible impl

Value value() const { return *this; }

[mhdawson]

We discussed this in the Node-api team meeting today, it seemed reasonable to us. Would you like to submit a pull request?

[mhdawson]

@neko-para maybe in https://github.com/nodejs/node-addon-api/blob/b347dbe35c85296bb983f25b27fa365a347bb41e/test/basic_types/value.cc