Consistent registry values and player changes

[Alvsch]

Description

• Now using IndexMap instead of HashMap for registry values so they have consistent ordering between runtime.
• player.world() can now be used instead of &player.living_entity.entity.world.
• Moved player.respawn() to world.respawn_player() as spawn_player is on world. (might reconsider)
• Preperations for multi-world support where the registry is involved.

Testing

• same biome every time
• no spawning in the ground
• multiple players work as before

Checklist

Things need to be done before this Pull Request can be merged.

• [x] Code is well-formatted and adheres to project style guidelines: cargo fmt
• [x] Code does not produce any clippy warnings cargo clippy