postFilter option in sample operator

## Proposal

Extend `sample` operator params with `postFilter` field, that would technically work the same as ordinary `filter` function, but evaluating right after `fn` does.

## Use case

```ts
/**
* Without postFilter
*/
guard({
clock: sample({
clock: chunksLoaded,
/** returns string or null */
fn: getSortingOrderFromChunks,
}),
/** passes through valid strings only */
filter: Boolean,
target: sortingUpdated,
});

/**
* With postFilter
*/
sample({
clock: chunksLoaded,
fn: getSortingOrderFromChunks,
postFilter: Boolean,
target: sortingUpdated,
});
```