allow derivers to return cleanup functions

[Rich-Harris]

fixes #2553. This makes it trivial to do things like this...

import { writable, derived } from 'svelte/store';

const hz = writable(60);

const heartbeat = derived(hz, ($hz, set) => {
  const interval = setInterval(() => {
    set(Date.now());
  }, 1000 / $hz);

  return () => clearInterval(interval);
});

...or, more practically, poll a URL for data where the variables can change. It doesn't attempt to fix your race conditions, but that's easy enough to do in userland.

[Rich-Harris]

A possible addition to this would be add a final argument to the cleanup function, so that if you (say) had a subscription manager of some kind that you only wanted to destroy when the store was no longer needed, you could do if (final) manager.destroy() or whatever. Is that worth including?