Add JS global variable source/target

[helgoboss]

See https://forum.cockos.com/showthread.php?t=259109

a) ... a source that emits the value of a JS global variable whenever it changes (combinable e.g. with the "OSC: Send message" target to send a numeric OSC message)

b) ... a target to set a JS global variable to the incoming control value (combinable e.g. with the "OSC" source to do this on incoming OSC messages)