feat: Add station searching with efficient geospatial index

[bkeepers]

This adds the ability to search for stations by proximity using the near and nearest functions:

import { near, nearest } from "@neaps/tide-database";

// Find all stations within 10 km of a lat/lon
const nearbyStations = near(
  { lat: 37.7749, lon: -122.4194 },
  { maxDistance: 10 },
);
console.log("Nearby stations:", nearbyStations.length);

// Find the single nearest station to a lat/lon
const nearestStation = nearest([-75.5, 22]);
console.log("Nearest station:", nearestStation);

The search uses geokdbush for efficient geospatial searching. The index is created at bundle time.