Barrier: Add a way to check if it's active asynchronously

In the docs, we have this example:

```ts
import { createBarrier } from '@farfetched/core';

const $authToken = createStore(/* ... */);

const authBarrier = createBarrier({
active: combine($authToken, (token) => isTokenInvalid(token)),
});
```

However, there is no way to check if the token is valid synchronously in Firebase, as getIdToken() always returns a promise.
https://firebase.google.com/docs/reference/js/v8/firebase.User#getidtoken

Maybe active should also accept an effect that returns a boolean?