QJSEngine/QQmlEngine: Support instantiating C++ classes in JavaScript using `new`

void ScriptContext::defineClass(const QString &className, QScriptEngine::FunctionSignature constructor)
{
    QScriptValue ctor = interpreter.newFunction(constructor);
    QScriptValue metaObject = interpreter.newQMetaObject(&QObject::staticMetaObject, ctor);
    interpreter.globalObject().setProperty(className, metaObject);
}

// scriptable.h
class InstantiableCppObj : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString name MEMBER m_name NOTIFY nameChanged)

public:
    Q_INVOKABLE InstantiableCppObj(const QString &name = QString(), QObject *parent = nullptr);

...
};

class CppUtils : public QObject
{
    Q_OBJECT

public:
    Q_INVOKABLE InstantiableCppObj* newInstantiableCppObj(const QString &name) { return new InstantiableCppObj(name); }
};


// main.qml
let cppObj = CppUtils.newInstantiableCppObj('Obj A')
console.log("Created a new C++ object from JavaScript called " + cppObj.name)

let cppObjB =     InstantiableCppObj('Obj B')                    // [1][3]
let cppObjC = new InstantiableCppObj('Obj C')                    // [2]
let cppObjD =     InstantiableCppObj.InstantiableCppObj('Obj D') // [1][4]
let cppObjE = new InstantiableCppObj.InstantiableCppObj('Obj E') // [2]