onDestroy callbacks are not called during SSR

[Rich-Harris]

Describe the bug

main-server.js re-exports onDestroy from main-client.js. This is wrong. That function looks like this:

export function onDestroy(fn) {
  if (!is_ssr) {
    user_effect(() => () => untrack(fn));
  }
}

Instead, we should export a server-side onDestroy directly from main-server.js, and it should run destroy callbacks once SSR is complete.

I also noticed that we're re-exporting createEventDispatcher, when that should (I think?) just be a no-op in SSR like onMount and friends.

Reproduction

In lieu of a repro I'll open a PR with a failing test

Logs

No response

System Info

next

Severity

annoyance