    line.closestPointTangent(point)

Return a line that is tangent to the line at the point that lies closest to `point`.

The tangent line starts at the identified closest point. The direction from `start` to `end` is the same as the direction of the line at the closest point.

If the two endpoints of the line both lie at the same coordinates, `null` is returned (it is impossible to determine the slope of a point). The `line.isDifferentiable()` function may be used in advance to determine whether tangents can exist for a given line.