Allow virtuals to be invoked in the definition of other virtuals

[sffc]

Summary

I sometimes use virtuals in the definition of other virtuals in my Mongoose schemas. This change allows virtuals to invoke other virtuals in TypeScript.

Examples

I have this code:

userSchema.virtual("tier").get(function() {
	// ...
	candidate = this.programModel?.tier_override;
	// ...
});

In this code, programModel is a virtual. However, compiling this code gives me an error:

src/user_model.ts:208:19 - error TS2339: Property 'programModel' does not exist on type 'Document<unknown, any, IUser> & IUser & Required<{ _id: ObjectId; }> & IUserMethods'.

208  candidate = this.programModel?.tier_override;
                      ~~~~~~~~~~~~

This PR fixes the error.

I have my virtuals defined in their own interface:

interface IUserVirtuals {
	// ...
	tier: string;
	programModel: IProgram | null | undefined;
	// ...
}

I use the following type shortcuts:

type UserModel = Mongoose.Model<IUser, {}, IUserMethods, IUserVirtuals>;
type UserSchema = Mongoose.Schema<IUser, UserModel, IUserMethods, {}, IUserVirtuals>;
type HydratedUser = Mongoose.HydratedDocument<IUser, IUserMethods, IUserVirtuals>;

The two {} are stand-ins for TQueryHelpers, which I don't currently use.

[vkarpov15]

Thanks