Deferred observers on components should fire after DOM updates

[Rich-Harris]

REPL. Paste the following into the JSON editor:

{
  "message": "hello everybody!"
}

Expected result: the speech bubble adjusts its size immediately.

Actual result: the speech bubble doesn't change its size unless you keep typing, and it only ever updates to the previous width of the <text> element.

[Rich-Harris]

Not limited to nested components: https://svelte.technology/repl?version=1.13.5&gist=0ce36a578a1b96b8ea4f732e4dfa7647

[Rich-Harris]

Ugh, pre and post were the wrong way round. Boy do I feel dumb