Support `for x in y` with `else`

From the [Svelte docs](https://svelte.dev/docs#template-syntax-each):

> An each block can also have an {:else} clause, which is rendered if the list is empty.

```svelte
{#each todos as todo}
	<p>{todo.text}</p>
{:else}
	<p>No tasks today!</p>
{/each}
```

Would be nice to have the same thing in Mayu.

```haml
= for todo in todos
  %p= todo.text
= else
  %p No tasks today!
```