Add ability to spawn actor from any logic

[davidkpiano]

This PR adds the ability for any actor logic to spawn an actor, which is a core tenet of the actor model.

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Latest commit: 9a695175695f56856323eb671688c51558347c84

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[davidkpiano]

There is a substantial problem with this PR that is somewhat mentioned in this comment here:

https://github.com/statelyai/xstate/blob/91549ce8a6a5105b3d354ca442ea5c058e58e206/packages/core/src/createActor.ts#L588-L592

When the parent actor gets stopped its children should get stopped too. There is no mechanism implemented here that would achieve this for this new functionality.

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