feat: allow signal effect to clean itself up inside callback function

[felixranesberger]

// this doesn't work, since the effect runs before the cleanup function is returned
const cleanup = imageSize.effect(() => {
    if (imageSize.value <= 0)
        return

    loadImage()
    cleanup() // throws error
})

// this works now
imageSize.effect((cleanup) => {
    if (imageSize.value <= 0)
        return

    loadImage()
    cleanup()
})

[jsebrech]

This has the effect of adding two listeners if called twice. Also, it doesn't quite feel "tiny". Perhaps for this use case a setTimeout is an adequate solution?

const cleanup = imageSize.effect(() => {
    if (imageSize.value <= 0)
        return

    loadImage()
    setTimeout(() => cleanup(), 0);
})