Provide convenience types for hasMany and belongsTo relationships

Which package(s) does this enhancement pertain to?

• @types/ember
• @types/ember__string
• @types/ember__polyfills
• @types/ember__object
• @types/ember__utils
• @types/ember__array
• @types/ember__engine
• @types/ember__debug
• @types/ember__runloop
• @types/ember__error
• @types/ember__controller
• @types/ember__component
• @types/ember__routing
• @types/ember__application
• @types/ember__test
• @types/ember__test-helpers
• @types/ember__service
• @types/ember-data
• @types/rsvp
• Other
• I don’t know

Please write a user story for this feature

As a developer, I want types importable from @ember-data/model that can be used to type model properties that represent async hasMany and belongsTo relationships.

These currently require importing from ember-data and using the somewhat esoteric PromiseManyArray (for hasMany) and PromiseObject (for belongsTo), e.g.

import Model, { hasMany, belongsTo } from `@ember-data/model`;
import DS from 'ember-data';

export default class MyModel extends Model {
  @hasMany('foo') foos!: DS.PromiseManyArray<Foo>;
  @belongsTo('bar') bar!: DS.PromiseObject<Bar> & Bar;
}

It would be nicer to have:

import Model, { hasMany, HasMany, belongsTo, BelongsTo } from `@ember-data/model`;

export default class MyModel extends Model {
  @hasMany('foo') foos!: HasMany<Foo>;
  @belongsTo('bar') bar!: BelongsTo<Bar>;
}

[jamescdavis]

https://github.com/DefinitelyTyped/DefinitelyTyped/pull/50044

[jamescdavis]

For the default async hasMany the type is:

DS.PromiseManyArray<T>

but for async: false it’s:

DS.ManyArray<T>

I was thinking of creating AsyncHasMany and SyncHasMany but since async: true is default, maybe the async type should just be HasMany? Any thoughts on names?