@joist/di: add multi providers.

[deebloo]

And multi providers. This comes from frameworks like angular where you can provide multiple instances of a particular provider. They get returned as an array. There are some use cases where this can be very useful but it would make a number of things more complex and could potentially hurt performance.

@injectable({
  multi: true
})
class ServiceA { }

const app = new Injector({
  providers: [
    { provide: ServiceA, use: ServiceA },
    { provide: ServiceA, use: ServiceA }
  ]
})

app.inject(ServiceA); // [ServiceA, ServiceA]