fix handling of polygons with holes

[ckp95]

The Rust version currently doesn't handle polygons with holes properly, because it doesn't account for interior rings.

The JS version:

import polylabel from "polylabel"

const polygon = [
    // exterior
    [
        [0, 0],
        [100, 0],
        [100, 100],
        [0, 100],
    ],

    // interior
    [
        [60, 60],
        [80, 60],
        [80, 80],
        [60, 80],
    ],
]

const result = polylabel(polygon, 1.0)
console.log(result)

[ 35.15625, 35.15625, distance: 35.13436819020658 ]

Diagram:

But the current Rust version gives 50, 50, i.e. just the centre of the outer ring:

This PR brings polylabel-rs in line with the JS implementation.