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

vlib/rand/wyrand/wyrand.v

@@ -250,9 +250,3 @@ pub fn (mut rng WyRandRNG) f64_in_range(min f64, max f64) f64 {
 	}
 	return min + rng.f64n(max - min)
 }
-
-// free should be called when the generator is no longer needed
-[unsafe]
-pub fn (mut rng WyRandRNG) free() {
-	unsafe { free(rng) }
-}