Thank you for creating this package!

Reproduction

I created a simple model but the .belongsToMany gives me the error Property 'belongsToMany' does not exist on type 'typeof ...

Steps to reproduce the behavior

1. Install piniaorm
2. Create a model (group.ts)

import { Model } from "pinia-orm";

import GroupMember from "./GroupMember";
import Member from "./Member";

export default class Group extends Model {
  static entity = "Group";

  static fields() {
    return {
      id: this.uid(),
      title: this.string(""),
      description: this.string(""),
      members: this.belongsToMany(Member, GroupMember, "group_id", "member_id"),
    };
  }
}

3. See that your editor (Webstorm in my case) says that belongsToMany does not exist.

Expected behavior

I would expect this.belongsToMany to exist and behave just like the original docs described it. https://vuex-orm.org/guide/model/relationships.html#many-to-many

Actual behavior

It says belongsToMany does not exist.

Additional information

If you didn't implement belongsToMany it might be good to put this in the README.md