Add support for ECMA6 Proxy to QJSEngine / v4vm

QJSEngine currently has some feature gaps compared to QtScript making it hard for many projects to move to QJSEngine. One of these capabilities is the option to trap access to properties dynamically, which in QtScript is provided by QScriptClass.

A standard compliant way to mitigate this without compromising the reasons for which v4vm was introduced would be to add support to ECMA 6 Proxy objects.

This is also important for keeping QJSEngine compatible with other runtime JS environments like V8, which also introduced support for ECMA 6 proxies recently.