Global Loot Tiers (A, B, C) for Better Structures & Mob Balancing

Currently, Minecraft loot tables are handled individually, making progression scaling difficult. I propose a global, data-driven Loot Tier System to standardize rewards based on risk.

Structure Tiers:
- minecraft:loot_tier_a (High Risk): Trial Chambers, Ancient Cities, Mansions.
- minecraft:loot_tier_b (Medium Risk): Mineshafts, Desert/Jungle Temples.
- minecraft:loot_tier_c (Low Risk): Villages, Ruined Portals, Shipwrecks.

Mob Tiers (minecraft:mob_loot_tier):
- Tier A: Bosses, Warden.
- Tier B: Piglin Brutes, Endermen.
- Tier C: Zombies, Skeletons, Creepers.

Structure-specific tags (like TC or AC) remain active so unique drops are preserved, but the core pool rolls from the global Tier first.

Config-Based Customization:
This tier system should be entirely manageable via JSON configs. If Mojang wants to buff mid-game loot, they adjust tier_b once instead of editing dozens of separate files. 

Benefits for Modders:
Modders can inject custom items into #minecraft:loot_tier_a with a single config line, automatically adding it to all tier-matching dungeons without causing file conflicts.

Examples of Use:
1. Want Netherite Ingots to rarely appear in all common structures (Villages, Portals)? Just add the item to "minecraft:loot_tier_c" config.
2. Want Zombies and Skeletons to drop Diamonds for a custom challenge? Add "minecraft:diamond" to "minecraft:mob_loot_tier_c". This updates all matching mobs instantly.