useObservables + model.use pattern

[guyca]

This PR adds two features that aim to help writing MVVM applications with Obsidian.

useObservers

This hook lets us observe multiple observables with a single method call.

Old	New
const [foo] = useObserver(fooObservable); const [bar] = useObserver(barObservable);	const {foo, bar} = useObservers({ foo: fooObservable, bar: barObservable });

Model base class

In MVVM, data and business logic in encapsulated in classes called "models". The models are made available to the views (typically functional components in React) via useViewModel hooks.

This PR implements an abstract model class that models can extend. It allows developers to easily observe all observables declared in the model using the model.use() method.

Example

1. Declare a model

import { Model } from 'react-obsidian';

class FooModel extends Model {
  public readonly foo = new Observable(1);
  public readonly bar = new Observable('bar');
}

2. Expose it via a graph

@Graph()
class FooGraph extends ObjectGraph {
  @Provides()
  fooModel() {
    return new FooModel();
  }
}

3. Inject the model to a viewModel hook, and use() it

const useFoo = ({ fooModel }: DependenciesOf<FooGraph, 'fooModel'>) => {
    const { foo, bar } = fooModel.use(); // { foo: number, bar: string }
    // Do something useful with foo and bar
  };

const useInjectedFoo = injectHook(useFoo, FooGraph);