Use separate types instead of generic types

[brichet]

Generic types can cause a lot of complex TypeScript errors that are difficult to resolve, so I recommend we avoid them unless they are necessary. This seems equivalent & easier to maintain:

interface IBaseChatMessage {
  type: 'msg';
  body: string;
  id: string;
  time: number;
  sender: unknown;
  attachments?: unknown[];
}

/**
 * The interface of a chat message in the chat model.
 */
export interface IChatMessage extends IBaseChatMessage {
  sender: IUser;
  attachments?: IAttachment[];
}

/**
 * The interface of a chat message in the YChat shared document.
 */
export interface IYMessage extends IBaseChatMessage {
  sender: string;
  attachments?: string[];
}

Originally posted by @dlqqq in #148 (comment)

For the context, this would be in replacement of the IChatMessage interface and the IYMessage interface.