Add lineClosest and polygonClosest

[HarryStevens]

See https://observablehq.com/@fil/distance-to-a-segment

[HarryStevens]

lineClosest: https://observablehq.com/d/07fca7f11189176e

[HarryStevens]

polygonClosest: https://observablehq.com/d/93845a6b6d0f0deb