Bevy_city traffic

[nuts-rice]

Objective

• Closes gap between bevy_city and it's inspiration

Solution

• Adds TrafficLight and associated enums for LightPhase and LightColor.
• TrafficLightMaterials and .glb asset
• TrafficPlugin
• fields in Car for handling traffic logic
• apply_traffic_rules for querying roads, cars and traffic light states.
• spawning logic for traffic lights at intersection

Testing

Right now it's feature gated and currently WIP.

• Cars do stop, but they should at minimum distance from one another
• light materials aren't showing on traffic light, it's just static gray pole rn
• should add car turning and tracking of cars egress of intersection
  Tests could be done for traffic logic, cars keeping distance at stop, possibly turning.

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Showcase

[alice-i-cecile]

My initial preference is to use a runtime toggle for this, not a feature flag. Make the case to me for why we should compile this out?

[nuts-rice]

i was mostly doing it for conveince. I can add it as toggle in settings instead

[alice-i-cecile]

Why is adding this as a feature to our example in particular useful?

[nuts-rice]

was trying to get tracing to work . I'll get rid of that

[alice-i-cecile]

This will be meaningfully slower than mutating the handle.

[alice-i-cecile]

Ditto here: don't use commands when you could just mutate.

[alice-i-cecile]

Comment is unclear (and it should be a doc comment).

[IceSentry]

I'm working on multiple changes that will heavily conflict with this. The traffic simulation I have planned is also pretty different to what you are doing here. I'm going to change how the hierarchy is spawned to make it much easier to work with and not make the cars children of roads.

Also, in general, I prefer keeping PRs small and focused so I would suggest making a separate PR that only adds the traffic lights without any simulation.