Function findBestWaypointInsertionIndex

findBestWaypointInsertionIndex(
    route: Route,
    existingWaypoints: WaypointLike[],
    newWaypoint: WaypointLike,
): number
Finds the best index to insert a new waypoint into an existing route.

It picks the position where the new waypoint fits most naturally along the route line:
1. It projects each existing waypoint onto its closest point on the route line.
2. It projects the new waypoint onto the route line.
3. It places the new waypoint by its position along the route.
Parameters
- route: Route
  The existing calculated route, a Feature with LineString geometry.
- existingWaypoints: WaypointLike[]
  The existing waypoints in order, from origin through intermediate waypoints to destination.
- newWaypoint: WaypointLike
Returns number
The insertion index. 0 inserts at the beginning, and existingWaypoints.length appends at the end.
Remarks
- If the route has fewer than 2 coordinates or there are fewer than 2 existing waypoints, it returns 0.
Example
```
// A route from [4.9, 52.3] to [5.0, 52.4], through [4.95, 52.35]
const waypoints: WaypointLike[] = [[4.9, 52.3], [5.0, 52.4]];
const newWaypoint: WaypointLike = [4.95, 52.35];

const insertIndex = findBestWaypointInsertionIndex(route, waypoints, newWaypoint);
// Returns: 1 (insert between origin and destination)

const updatedWaypoints = [
  ...waypoints.slice(0, insertIndex),
  newWaypoint,
  ...waypoints.slice(insertIndex)
];
```