Add concurrency helpers for EffectHandler and EventSource

[kmcbride]

Effect Router

These changes introduce support for asynchronous effect route handlers by wrapping the existing EffectHandler route to create and callback from a new task. It currently supports routing to async functions that return a single event, a sequence of events, or nothing at all.

Additionally, a .receiveOn(queue:) synchronization helper has been added so that callback events can be moved to the loop queue — a necessity if routing to event-producing async functions and not using MobiusController, but also generally useful to all effect handlers that would otherwise need to dispatch their callbacks on a specific queue.

let effectRouter = EffectRouter<EffectType, EventType>()
    …
    // Routing to async func references
    .routeCase(EffectType.caseName1).to(asyncFunc)
    .routeCase(EffectType.caseName2).to(asyncThrowsFunc)
    // Routing to async func that produces events that need to be received on a specific queue
    .routeCase(EffectType.caseName5).receiveOn(queue: loopQueue).to(asyncEventProducingFunc)
    // Routing to async closures
    .routeCase(EffectType.caseName6).to { @MainActor (effectAssociatedValue: EffectAssociatedValueType) async in
        await someAsyncCode(input: effectAssociatedValue)
    }

---

Event Source

These changes also introduce support for adding an AsyncSequence to the CompositeEventSourceBuilder.

let eventSourceBuilder = CompositeEventSourceBuilder<EventType>()
    …
    // The `receiveOn consumerQueue:` parameter is intended to be used when building a `MobiusLoop`
    // so that events can be sent back to the loop queue from the task
    .addEventSource(eventProducingSequence, receiveOn: loopQueue)
    // It can safely be omitted when building a `MobiusController`
    // because queue switching is handled automatically
    .addEventSource(eventProducingSequence)