Add support for specifying sync/async for @observes.

[rwjblue]

Ember 3.13 added support for specifying if an observer should be fired synchronously or asynchronously. Future versions of Ember will default to firing these observers asynchronously, but you can manually specify if you would like sync or async behaviors.

import EmberObject from '@ember/object';
import { observes } from '@ember-decorators/object';

class Foo extends EmberObject {
  @observes({
    dependentKeys: ['foo'],
    sync: false, // run asynchronously regardless of the application wide default
  })
  bar() {
    //...
  }
}

[pzuraq]

We also need to add the sync option to @unobserves, sync and async observers get added and removed separately from each other so you need to specify the option when removing as well.