js: fix rand build, properly use key values on map, add rand.string (#12020)

vlib/rand/rand.js.v

@@ -0,0 +1,17 @@
+module rand
+
+// init initializes the default RNG.
+fn init() {
+	default_rng = new_default()
+}
+
+pub fn string(len int) string {
+	result := ''
+	#
+	#const characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
+	#const charactersLength = characters.length;
+	#for (let i = 0;i < len.val;i++)
+	#result.str += characters.charAt(Math.random() * charactersLength);
+
+	return result
+}