Add `atlas::event::scene_transition` event for multi-scene contexts

[SpinnerX]

Incoming plans in extending the event bus to handle coordinating when to transition scenes within a multi-scene context.

Objectives for Extending the Event bus:

• New Scene Transition Event
• Introduce multiple types of events (single-use execute events, time interval events, etc.)
• Potential event queue system for single-time execution events. (considering).

I plan to flush this out to make sure we can solidify the design for scene transitioning.