Update index.d.ts (#25313)

Added `async` and `defer` as optional properties of the `<Helmet>` component.

types/react-helmet/index.d.ts

@@ -7,9 +7,11 @@
 import * as React from "react";
 
 export interface HelmetProps {
+    async?: boolean;
     base?: any;
     bodyAttributes?: Object;
     defaultTitle?: string;
+    defer?: boolean;
     encodeSpecialCharacters?: boolean;
     htmlAttributes?: any;
     onChangeClientState?: (newState: any) => void;