[feature] watchOnce

Sometimes it's good to have a watch that will react on change only once. I've made a function for this:

```ts
export const watchOnce = <T>(
  target: WatchTarget<T>,
  cb: (res: T, prev?: T) => any
) => {
  const unwatch = watch(target, (res, prev?: T) => {
    unwatch();
    cb(res, prev);
  });
  return unwatch;
}
```
It returns a function to disable the watch manually.

WDYT?