[core] Add waitFor helper

[davidkpiano]

This PR adds the opt-in waitFor(...) helper function, that asynchronously waits for the emitted value of an actor to satisfy a predicate before returning that value.

See discussion here: https://discord.com/channels/795785288994652170/955085268677963776

Basic usage:

import { waitFor } from 'xstate/lib/waitFor';

// ...
const loginService = interpret(loginMachine).start();

const loggedInState = await waitFor(loginService, state => state.hasTag('loggedIn'));

loggedInState.hasTag('loggedIn'); // true

cc. @mattpocock @gavination

TODO:

• Add changeset

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🦋 Changeset detected

Latest commit: 281f1fe

The changes in this PR will be included in the next version bump.

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xstate	Minor

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[Andarist]

was this change needed?

[davidkpiano]

Tests were failing without it locally

[Andarist]

what about actors that do not emit their current state at subscription time? or are actors supposed to always do that? Note that today an actor made out of an observable might not emit immediately because the emit time is controlled by the observable itself

Should we run predicate with .getSnapshot() first? With that I'm a little bit worried about handling non-started interpreters as they would instantiate their .initialState and that is not a pure operation right now 😢

[davidkpiano]

For now, let's have the behavior as-is - if actors (like observables) do not emit state upon subscription, this utility should respect that.

That avoids the impure initial state problem, too 👍