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

vlib/rand/wyrand/z_wyrand.c.v

@@ -0,0 +1,7 @@
+module wyrand
+
+// free should be called when the generator is no longer needed
+[unsafe]
+pub fn (mut rng WyRandRNG) free() {
+	unsafe { free(rng) }
+}