change type of index from string to number

Author: Feiyang1

packages/analytics-types/index.d.ts

@@ -206,7 +206,7 @@ export interface Item {
   item_variant?: string;
   price?: Currency;
   quantity?: number;
-  index?: string;
+  index?: number;
   coupon?: string;
   item_list_name?: string;
   item_list_id?: string;

packages/firebase/index.d.ts

@@ -5050,7 +5050,7 @@ declare namespace firebase.analytics {
     item_variant?: string;
     price?: Currency;
     quantity?: number;
-    index?: string;
+    index?: number;
     coupon?: string;
     item_list_name?: string;
     item_list_id?: string;