Support model creation w/o registry

[ampatspell]

@name('other') // for @model / @models .named()
class Other extends Model {

  constructor(owner, props) {
    super(owner);
  }

}

export default class Thing extends Model {

  @model()
    .named(({ type }) => type === 'other' ? Other : Foo)
    .mapping(thing => ({ })
  other;

}

• create model using provided factory (@model and @models)
• support model recreation based on model name which is set using @name decorator