Feature: Throw an error if model class has multiple foreign keys referencing the same model

[RobinBuschmann]

sequelize-typescript should throw an error if it finds multiple foreign keys referencing the same model like:

@Table
export class FriendList extends Model<FriendList> {

  @ForeignKey(() => User)
  @Column
  UserId: number;

  @ForeignKey(() => User)
  @Column
  FriendId: number;
}

This should be because it cannot infer which is the actual foreignKey and which one is the otherKey.

[katanacrimson]

This would be very, very valuable on more complex schemas.

Silently ignoring the other key would not have gone noticed if I wasn't looking directly at the generated schema just now, and it took me some digging to remember why this was a problem. This is an error case and warrants throwing an error as such.