AsyncQueuer: consider adding onStart callback for when an item transitions from pending to active

[sasha-morozov]

Noticed that AsyncQueuer has per-item callbacks for every post-execution outcome - onSuccess, onError, and onSettled, but there doesn't seem to be an equivalent for when an item begins executing. The only way to observe that moment today is through onItemsChange, which fires at the queue level on any mutation.

Would it make sense to add, for instance, an onStart callback that fires just before an item transitions from pending to active - similar to how TanStack Query's useMutation does with onMutate callback? E.g. onStart?: (item: TValue, queuer: AsyncQueuer<TValue>) => void - consistent with the existing callback signatures.

Or is there a reason this wasn't included?