feat: enhance IsInViewport and useIntersectionObserver

[max-got]

…with observer control methods and once option.

• Added methods to IsInViewport class: stop(), pause(), resume(), and isActive to manage the intersection observer.
• Updated useIntersectionObserver to include an optional 'once' parameter that stops observing after the first intersection.

As i way using IsInViewport and useIntersectionObserver i often found the need to unobserve on intersecting. Especially for classic fade in animations and so on. Furthermore i was confused that i cant handle the IsInViewport utility correctly. Therefore i added these two enhancements.

Just lmk if that is not desired or if anything is not correctly implemented. I have opted for once naming scheme. Alternatives would be: triggerOnce, unobserveOnEnter, ...

Example:

<script lang="ts">
	import { IsInViewport } from "runed";
	let targetNode = $state<HTMLElement>()!;
	const inViewport = new IsInViewport(() => targetNode, {
		once: true
	});
	
	inViewPort.stop();
	inViewPort.isActive;
	//...
</script>
<div bind:this={targetNode} class="transition" class="transition-opacity {hasBeenSeen ? 'opacity-100' : 'opacity-0'}">
	I'll fade in once when first visible, then stop observing
</div>

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