transitions in await blocks

[Rich-Harris]

Follow-up to #952. This currently doesn't work:

{{#await promise}}
  <p transition:fade>loading...</p>
{{then value}}
  <p class='content' transition:fade>{{value}}</p>
{{catch error}}
  <div class='toast' transition:fly='{y: 100}'>{{error.message}}</div>
{{/await}}