fix(useIntersectionObserver): update syntax to handle stricter types

[cyrilf]

References:

• vueuse issue 🐛
• Nuxt v4 issue

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Description

Due to stricter types planned on the upcoming v4 of Nuxt, the demo example needs to be updated.
You can look at this Nuxt issue for reference.
The Typescript issue occurs when future: { compatibilityVersion: 4 }, is applied in the nuxt.config.ts file.

The change is not Nuxt related though, it concerns anyone.
It's simply applying an extra check on the IntersectionObserverCallback argument to ensure we handle the possibility of having an entry being undefined instead of immediately de-structuring.

Additional context

Reproduction link of the issue

As a bonus, it makes the code a bit more beginner-friendly (we could even use the name entries and access it via entries[0]?.isIntersecting but I think that's out-of-scope from vueuse, as all information are already available on MDN.

I made a choice towards visibility but feel free to choose another way of writing it

For instance: (for brevity but less beginner-friendly)

- targetIsVisible.value = entry?.isIntersecting || false
+ targetIsVisible.value = !!entry?.isIntersecting