Update index.d.ts

An overload of `app.use`, see docs for `app.mountpath` here (sub-app): https://expressjs.com/en/api.html#app.mountpath

types/express-serve-static-core/index.d.ts

@@ -44,6 +44,7 @@ export type RequestHandlerParams = RequestHandler | ErrorRequestHandler | Array<
 export interface IRouterMatcher<T> {
     (path: PathParams, ...handlers: RequestHandler[]): T;
     (path: PathParams, ...handlers: RequestHandlerParams[]): T;
+    (path: PathParams, Application): T;
 }
 
 export interface IRouterHandler<T> {