Expose set_raf in Svelte 5

[grischaerbe]

Describe the problem

The usage of requestAnimationFrame-based tweened and spring fails in XR since an XRSession provides its own requestAnimationFrame.

With Svelte 4, Threlte was able to provide the appropriate requestAnimationFrame via set_raf (implemented here) but this ability is currently missing in Svelte 5.

Describe the proposed solution

Expose a method to set the driver of tweened and spring and other motion-based utilities, possibly per-instance.

Importance

nice to have

[paoloricciuti]

Can't you import raf and update the tick function?