Port turf-union

[trasch]

• https://github.com/nighthawk/ASPolygonKit/blob/main/Sources/ASPolygonKit/MKPolygon%2BUnion.swift
• Use Martinez-Rueda-Feito polygon clipping algorithm nighthawk/ASPolygonKit#3
• https://github.com/luizbarboza/polyclip-ts/tree/main/src
• https://github.com/Turfjs/turf/tree/master/packages/turf-union