Reactive components prototype

[JMS55]

Objective

• Initial prototype for reactivity in Bevy

Solution

• A new ReactiveComponent type that automatically inserts and updates a component of type Output on the same entity whenever the source entity's component of type Input changes (as detected by change detection ticks)

Implementation issues

• There's a single place where reactive components get updated (the Last schedule). Ideally it would be an immediate reaction like how observers work.
• The update_reactive_components driver system loops over every single expression any time an expression changes. It would be ideal if we could track affected components, and only re-run expressions that would be affected. But for the initial prototype, performance isn't too important anyways.

Not implemented/tested

• ReactiveComponents that can add/remove the component, instead of just modifying the value.
• Chaining reactions
• Expressions that depend on multiple components from an entity
• Expressions that depend on multiple entities
• Expressions that depend on resources
• Expressions that depend on queries or other complicated sources of data
• Reactive entity tree structures (e.g. a reactive version of WithChildren/WithChild, using the Name component as a key)

[JMS55]

I think template + diffing approaches are the better move here. Change detection is too difficult to get working well I feel.