Correct typings for runes (#34841)

* Update runes-tests.ts

* Update runes types

Correct to account for width param

types/runes/index.d.ts

@@ -5,7 +5,7 @@
 
 declare function runes(text: string): string[];
 declare namespace runes {
-    function substr(text: string, index: number): string;
+    function substr(text: string, start: number, width?: number): string;
 }
 
 export = runes;

types/runes/runes-tests.ts

@@ -2,3 +2,4 @@ import runes = require('runes');
 
 runes('hello world 😁');
 runes.substr('hello world 😁', 1);
+runes.substr('hello world 😁', 1, 5);