export IRouterHandler too (#13840)

Export IRouterHandler like IRouterMatcher.

I' like to use and extend it in global middlewares.

express/index.d.ts

@@ -59,6 +59,7 @@ declare namespace e {
     interface Handler extends core.Handler { }
     interface IRoute extends core.IRoute { }
     interface IRouter<T> extends core.IRouter { }
+    interface IRouterHandler<T> extends core.IRouterHandler<T> { }    
     interface IRouterMatcher<T> extends core.IRouterMatcher<T> { }
     interface MediaType extends core.MediaType { }
     interface NextFunction extends core.NextFunction { }