[New Feature] Unified API to retrieve/manage UGC (user-generated content)

[SFGrenade]

Ideally it would be a class/method that returns the UGC folder for the mod.
2 options of implementing it:

1. "Dumb" function

class UGCHelper {
    // UGCRoot to be determined
    public static string GetUGCPath(string modName) => Path.Combine(UGCRoot, modName);
}

2. Mod-specific

public abstract class Mod : Loggable, IMod {
    ...
    // UGCRoot to be determined
   public string GetUGCPath() => Path.Combine(UGCRoot, GetName());
}