types: avoid automatically inferring lean result type when assigning to explicitly typed variable

[vkarpov15]

Fix #14697

Summary

With the way that lean() is currently implemented, the following code unexpectedly compiles

export interface IUser {
  name: string;
  createdAt: Date;
  updatedAt: Date;
}
export interface IUserVirtuals {
  id: string;
}
type UserModelType = Model<IUser, {}, {}, IUserVirtuals>;
export type UserInstance = InstanceType<UserModelType>;


let user: UserInstance | null = null;
// Compiles even though `lean()` but `user` is hydrated doc
user = await UserModel.findOne().lean();

That's because TypeScript automatically infers the generic param to lean() if user has an explicit type.

In order to support this, I needed to change the default value of RawDocType param to Query<> to unknown. When RawDocType was set to DocType by default, that would break when DocType was set to be a hydrated document.

Examples