Support LineString/MultiLineString base data

Since we don't use the actual type but rather collapse the multis to non-multis, I figured it was best to use a simple string and make it clear that we shouldn't expect every GeoJSON type to show up. This also gives us flexibility to have specific styles for things like "sidewalk" or "intersection", which we might want to style different from a generic, fallback "linestring" or "point" style.