Add item_model data component support for tokens

[PurpleGale]

This PR adds support for the new Minecraft item_model data component (introduced in 1.21.4) to the token system, while fully preserving the existing custom_model_data functionality.

Changes

• TokenData.java — Reads the new item-model config block and applies it via DataComponentTypes.ITEM_MODEL using a validated NamespacedKey.
• config.yml — Adds item-model.enabled and item-model.value under every token entry.
• ToolStats.java — Bumps CONFIG_VERSION from 17 to 18.
• ConfigUpdater.java — Wires up the 17 → 18 migration path.
• Version18.java (new) — Config updater that injects item-model defaults into existing user configs during automatic migration.

Backward Compatibility

• Existing custom-model-data behavior is completely untouched.
• Both custom-model-data and item-model can be used independently or together.
• Old configs are automatically migrated to version 18 on first load.

Usage Example

tokens:
  data:
    player-kills:
      item-model:
        enabled: true
        value: "nexo:my_custom_token"