Add a line option to the area mark

[Fil]

this creates a derived line on x2/y2 which consumes the stroke; this is the "top" line for areaY, and the "right-most" line on areaX. It might be a bit unexpected or useless on area, needs experimenting to see if we must ignore it.

Typical use case https://observablehq.com/@recifs/spectrum-plotter-basic