make crossfade fallback param optional

[macmillen]

It would be nice to mark the fallback param of the crossfade function from "svelte/transition" as optional to avoid passing an empty function when no fallback is desired.

before:

  const [send, receive] = crossfade({
    duration: 600,
    fallback: () => ({}),
  });

after:

  const [send, receive] = crossfade({ duration: 600 });

Notes

I think this was also intended by the author because there is already a null check for fallback.
return fallback && fallback(node, params, intro);

[Conduitry]

This type fix has been included in 3.38.0 - thanks!