feat: add Firestore vector search integration and noun type-based search enhancements

Introduced Firestore vector search integration using `@firebase/firestore-vector-search`. Added support for restricting searches by specific noun types to optimize performance and relevance. Updated `README.md` to document the new functionality and usage examples.

src/hnsw/hnswIndex.ts

@@ -364,6 +364,13 @@ export class HNSWIndex {
     return this.nodes.size
   }
 
+  /**
+   * Get the distance function used by the index
+   */
+  public getDistanceFunction(): DistanceFunction {
+    return this.distanceFunction
+  }
+
   /**
    * Search within a specific layer
    * Returns a map of node IDs to distances, sorted by distance