Fix the Function interface (#11269) (#11293)

Author: falsandtru

lib/lib.d.ts

@@ -260,6 +260,9 @@ interface Function {
       */
     bind(this: Function, thisArg: any, ...argArray: any[]): any;
 
+    /** Returns a string representation of an object. */
+    toString(): string;
+
     prototype: any;
     readonly length: number;