Add Pydantic models for save data

[quae-dev]

Define Pydantic models for each of the following formats:

• Save files (user*.dat)
• Shared data (shared.dat)
• Autosaves (restoreData*.dat)

This will allow for in-depth data validation as well as rich, typed property access.

One potential downside is that this will make the library fundamentally incompatible with future changes to the save file structure.