Easy :) a small image of a black line plus a transparent line underneath, drawn tiled, at viewport scale and about 0.1 opacity, in the DrawGUI Event of a GUI object I always keep on the top layer
I think I used a 2x2px or 4x4px image like that, just in case using a non-uniform sprite would cause any scaling issues (it would probably be fine though, GMS2's tiled drawing is good) :)
Then if you wanted to get fancy with it, give players an option for rotating the scanlines 90 degrees, since some people prefer their scanlines vertical for some reason