Allow Svelte components to be slot roots

https://svelte.technology/repl?version=2.13.2&gist=1a69018031dc8c34d013de01a70579a1

Instead of this:

```
<Layout>
  <Component slot="my-slot" />
</Layout>
```

I have to do this:

```
<Layout>
  <div slot="my-slot">
    <Component />
  </div>
</Layout>
```

This adds an unnecessary DOM layer and possibly means adjusting CSS.