Improve useNearScreen implementation

[josepot]

There is a small problem with the current implementation of useNearScreen, which is that if a user navigates to a different page without having seen all the all the "observed elements", I believe that can create a small memory leak because those instances of IntersectionObserver are never disconnected/unobserved.

A way to address this is to just use one instance of IntersectionObserver and to unobserve the elements either once they are intersected or when they are unmounted (using the cleanup function of useEffect). I think that this change could improve the performance of this hook.

Also, notice that with these changes if the polyfill is needed, then its download would start significantly earlier. Therefore, decreasing the risk of an element being intersected before its observer is created. Unless I'm mistaken, with the current implementation the download of the polyfill would start after the root component is mounted. With these changes the download should start while the "App/Main" js asset is being parsed.

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