Create a generic `content` registration for Plugins

With the intent that they can be read and visualised with an introspection.

Bonus points if you kan bind methods to the introspection: `hasKeyItem()`, `upgradeLevel()` and such

Look for existing plugin registration code in the codebase, likely in a directory like 'src/plugins' or 'src/core'. Identify where content types are currently defined and how introspection is handled. The goal is to design a generic registration system that allows plugins to declare content and bind methods like hasKeyItem() and upgradeLevel() for introspection.

索引モデルが issue の本文から書いたものです。