bug(@turf/midpoint): altitude (z-coordinate) not interpolated for 3D points

Bug Report

Package: @turf/midpoint

Description

When both input points carry a z-coordinate (altitude/elevation), midpoint() returns a point whose z equals the origin's z unchanged instead of the linear average (z1 + z2) / 2.

Reproducible example

import { midpoint } from '@turf/midpoint';
import { point } from '@turf/helpers';

const p1 = point([0, 0, 100]);   // altitude 100
const p2 = point([10, 0, 200]);  // altitude 200
const mid = midpoint(p1, p2);

console.log(mid.geometry.coordinates[2]);
// Actual:   100   ← same as p1's z
// Expected: 150   ← (100 + 200) / 2

Root Cause

midpoint() delegates to destination(origin, dist/2, heading). destination() propagates only the origin's z-coordinate unchanged (see source):

if (coordinates1[2] !== undefined) {
  return point([lng, lat, coordinates1[2]], options.properties);
}

Because midpoint passes point1 as the origin, the result always gets z = z1, discarding z2 entirely.

Expected Behaviour

The midpoint of two 3D points should carry z = (z1 + z2) / 2.

Fix

Extract both z-values and average them when both are defined; fall through to current behaviour (no z) when either is absent.