consider adding Line.interpolate

[ianstormtaylor]

In my case I'm trying to quickly interpolate between two line segments. Ideally it would be as easy as Segment.interpolate(a, b, 0.25).

I thought I might be able to use Polygon.interpolate instead, but the TypeScript types complain about that. And using Point.interpolate for the individual points gets tedious. Maybe I'm doing something wrong or missing an easier approach though!