Added SerializableDifficultyDef

[Nebby1999]

A piece of code that was originally created on MSU, the SerializableDifficultyDef simply acts, as the name implies, as a Serializable version of DifficultyDef.

This can be used to pre-create difficultyDefs on thunderkit mods and then have them added by R2API's AddDifficulty() methods. The "finalized" difficultyDef is a property on the ScriptableObject and the DifficultyIndex can be obtained from another property on the ScriptableObject itself.

[Nebby1999]

Turns out the 3 methods that add Items, Equipments and Elites to content additions would technically end up creating a contentPack for r2api, latest commit fixes this issue.

Tbh now i very much regret making those methods, but now i do not know if there are already mods that use ContentAddition for adding those instead of the API's there are... oh well 💀