Refactor new ECS rubble further

[Cervator]

After merging #593 the "golden test asteroid" that spawns in front of the starting station now uses ECS-based rubble creation! There are some outstanding next steps however:

• [ ] The rubble starts with no momentum added - they should be flying all over the place (if they get bumped they also start acting bizarrely, like flying in circles)
• [x] The rubble is invincible (same state before #593, normal asteroids are fine) and needs to be destructible
• [ ] The tubble goes straight to the tiniest size - a larger asteroid should first break into smaller pieces
• [ ] Smaller pieces should probably also have a chance to drop loot, but destroyed tiny rubble pieces should not?
• [ ] Maybe really a separate item: regular asteroids should be updated to the new system - but this may involve a fair bit more work including spawning, despawning, and random starting momentum/movement if any

If anybody new wants to work on this feel free - it isn't necessarily a Good First Issue since the topic is complex, but the area is relatively tiny and approachable and we can give more directions here or on Discord as needed in response to well-informed questions :-)

[BenjaminAmos]

This isn't closed yet! Not all the issues have been solved.