Add `assertEventType` helper

[tom-sherman]

Adds a helper function that can be used as an invariant to narrow the type of events that are passed to actions, services, guards, etc.

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[Andarist]

I don't mind introducing such a helper but I'm worried that it will give users a false sense of safety. After all - this can throw, how do you respond to the thrown error?

[tom-sherman]

Good point, I wonder if that can be communicated (or is already communicated) in the name and API. I only see this useful when using typescript and in Dev mode.

[tom-sherman]

Maybe an alternative could be a type predicate function, not as ergonomic though IMO - not much benefit over doing the conditional yourself.

[davidkpiano]

Superseded by assertEvent(...) https://stately.ai/docs/typescript#asserting-events