Add system_random and random_bytes for js target

core/math/rand/system_js.odin

@@ -0,0 +1,11 @@
+package rand
+
+foreign import "odin_env"
+foreign odin_env {
+	rand :: proc "contextless" () -> f64 ---
+}
+
+@(require_results)
+_system_random :: proc() -> u64 {
+	return u64(rand() * 0x1fffffffffffff)
+}