Support for `Relation`.

[haejunejung]

It provides support for Relation.

It wrappers type for relation type definitions in entities. It is used to circumvent ESM modules circular dependency issue caused by reflection metadata saving the type of the property.

@link
https://github.com/typeorm/typeorm/blob/master/src/common/RelationType.ts

import { Entity, PrimaryGeneratedColumn, OneToOne } from "typeorm";

@Entity
export class Profile {
  @PrimaryGeneratedColumn()
  id: number;

  @OneToOne(() => User, user => user.profile)
  user: Relation<User>;
}

@Entity
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @OneToOne(() => Profile, profile => profile.user)
  profile: Relation<Profile>
}