Add a new type definition for Randomstring (#9252)

* Create randomstring-tests.ts

* Create randomstring.d.ts

randomstring/randomstring-tests.ts

@@ -0,0 +1,18 @@
+/// <reference path="randomstring.d.ts" />
+
+import * as randomstring from "randomstring";
+
+randomstring.generate();
+
+randomstring.generate(24);
+
+randomstring.generate({
+    length: 12,
+    readable: true,
+    capitalization: "",
+    charset: "alphabetic"
+});
+
+randomstring.generate({
+    charset: "abc"
+});