function offset(c1, distance, bearing, radius) {
  radius = radius || DEFAULT_RADIUS;
  const lat1 = toRadians(c1[1]);
  const lon1 = toRadians(c1[0]);
  const dByR = distance / radius;
  const lat = Math.asin(
    Math.sin(lat1) * Math.cos(dByR) +
      Math.cos(lat1) * Math.sin(dByR) * Math.cos(bearing),
  );
  const lon =
    lon1 +
    Math.atan2(
      Math.sin(bearing) * Math.sin(dByR) * Math.cos(lat1),
      Math.cos(dByR) - Math.sin(lat1) * Math.sin(lat),
    );
  return [toDegrees(lon), toDegrees(lat)];
}
Returns the coordinate at the given distance and bearing from `c1`.

@param {import("./coordinate.js").Coordinate} c1 The origin point (`[lon, lat]` in degrees).
@param {number} distance The great-circle distance between the origin
    point and the target point.
@param {number} bearing The bearing (in radians).
@param {number} [radius] The sphere radius to use.  Defaults to the Earth's
    mean radius using the WGS84 ellipsoid.
@return {import("./coordinate.js").Coordinate} The target point.