feat: add onFrame lifecycle function

[Rich-Harris]

Just an idle thought I had while washing the dishes: should we have an onFrame function? It would mean for example that this demo could be written like this:

$effect(() => {
  const context = canvas.getContext('2d');

  onFrame(() => {
    paint(context, Date.now());
  });
});

Alternatively we might want to have some sort of reactive time primitive (other than SvelteDate) so that it could be used in deriveds too. Either way not wedded to it, just an idle thought that I thought was worth a PR

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14594-svelte.vercel.app/

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[dummdidumm]

While it's nice, this does feel more of a thing for a utility or animation library. I wouldn't call it a lifecycle function at least and expose it through svelte/reactivity instead.

[ottomated]

Adjacent to #12441

[trueadm]

Adjacent to #12441

I'm not sure it is at all. You definitely don't want to be updating the UI with Date.now() every frame, that's going to put a lot of pressure on the runtime for no real benefit. I'd imagine you'd only want to update the time in relation to your UI requirements, such as every second or minute. This is very much a user-space problem rather than a problem that Svelte should even attempt to solve.

[trueadm]

I've been thinking more about this and I think its the name I don't like. Maybe we could call it onAnimationFrame as this is really where you're meant to handle scheduling of animation related things. I worry about onFrame as it feels very close to onMount in that people will just want something to happen on the next frame, but in reality it continuously happens.