Use computedFn as annotation in make(Auto)Observable

[grasshoppergn]

It seems that the only way now to use computedFn in model classes is

class Model {
  getSomething = computedFn((params) => "return something");
  constructor() {
    makeAutoObservable(this, {
      getSomething: false,
    });
  }
}

So it looks quite convenient to allow computedFn to be used as an annotation, so it becomes

class Model {
  getSomething(params) {
    return "return something";
  }
  constructor() {
    makeAutoObservable(this, {
      getSomething: computedFn,
    });
  }
}

Can also be done with bounds / not bound versions

[whalemare]

Duplicate of #236

[grasshoppergn]

@whalemare is it? #236 doesn't automatically make it useful in makeAutoObservable...