Implement exhaustive event handling for Fiber/Script-based Entity Threads

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Our current approach for event handling is to capture a single eligible event object in one pass, then handle the event in a once-per-frame 'step' of the Entity Thread. -- Instead, we should provide the ability to handle multiple events in a single frame, updating scripts as events are dequeued, rather than deferring the work.

Threads using this exhaustive approach could then use

co_await next_frame;

or similar, if this exhaustive capturing behavior was not intended. -- e.g. to prevent handling multiple input events on a single frame.

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Implemented initial support for this. We'll need to revisit this when script coroutines become scheduled, rather than executing on the main thread.