Feature: Support for loading components by promise

Hi, if you do like this

```
<Loadable loader="{import('./Test.svelte')}" delay="{delay}">
  <div slot="success" let:component>
    <svelte:component this="{component}" {...params} />
  </div>
</Loadable>
```

It won't load the component and will give error like; `component.then is not a function`
Maybe you can detect is it a promise in loader and if it is, it can await it to load?
