Add a new type definition for Randomstring (#9252)

* Create randomstring-tests.ts

* Create randomstring.d.ts

randomstring/randomstring.d.ts

@@ -0,0 +1,19 @@
+// Type definitions for randomstring 1.1.4
+// Project: https://github.com/klughammer/node-randomstring
+// Definitions by: Isman Usoh <https://github.com/isman-usoh/>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+
+declare namespace Randomstring {
+    interface GenerateOptions {
+        length?: number;
+        readable?: boolean;
+        charset?: string;
+        capitalization?: string;
+    }
+
+    function generate(options?: GenerateOptions): string;
+}
+
+declare module "randomstring" {
+    export = Randomstring;
+}