Provide types for the custom property

[vincentharrius]

On many of the core types, the custom property has the type any. It would be helpful when developing in typescript to determine the type by using generics.

Current:

export interface Player {
    // ...
    custom: any;
}

Proposal:

export interface Player<T = any> {
    // ...
    custom: T;
}

This would be done for the following interfaces:

• Game
• Player
• Spectator
• Turn