HorseMixin

I was originally just reporting the tracked data handler, but it's always good to improve compatibility and safety.

1. adds a new tracked data handler
   This is extremely unsafe and prone to load-order dependent issues that are impossible to debug.
   You should instead add a regular field to the mixin, and sync it manually with a custom packet.

2. Overrides removeWhenFarAway
   Method overrides in mixins are worse than an @Overwrite, because it's the same, but silent. You should instead inject to the parent class and check instanceof AbstractHorse.

3. cowboySpawned not marked as @Unique
   This means your field may conflict with and overwrite/be overwritten by another mod's.
   It also makes your mod harder to track down if it ends up in an error. Unique fields will be prefixed with the mod ID automatically.

4. methods in HorseHat are not prefixed
   These method overrides may conflict with methods from another mod. These can't be marked Unique because they need to match the names in the interface. This is usually resolved my prefixing injected interface method names with your mod ID. ex. biome_makeover$hasHat

2, 3, and 4 likely also apply to other mixins, but HorseMixin is all I've looked at.