Add support for undocumented global function `ObjFun()`

[lvcabral]

This function was discovered by the community and was discussed here:
https://community.roku.com/t5/Roku-Developer-Program/objFun-no-fun/td-p/306374

a valid code is this:

    di = createObject("roDeviceInfo")
    iface = getInterface(di, "ifDeviceInfo")
    print objFun(di, iface, "getModel")
    print objFun(di, iface, "canDecodeVideo", {codec: "mpeg2"}).result