I dont know if there exist an extension like that. But I could share how I idle.. I use a corsair keyboard (k95, k55) with macro functionality and a macro that auto press forward and backward (w,s) once in a while. That also works on my roccat kone. The good thing is that you could unplug the keyboard and use it on another computer after setting it up, probably same with the mouse. The problem ofcourse is that it can only be used on the active window. In that case you have to add alt+tab in the macro and that could be annoying.