Add bind:scrollWidth & bind:scrollHeight properties

[nounder]

Describe the problem

Svelte has this wonderful feature that allows to create binding for {client,offset,content}{Width,Height} and other related properties. However, it does not support scroll{Width,Height}. Binding to those values would be useful for creating carousels, sliders, and other scroll-based components.

svelte/packages/svelte/src/compiler/phases/bindings.js

Lines 149 to 181 in 3ee8e0b

	// dimensions
	clientWidth: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},
	clientHeight: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},
	offsetWidth: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},
	offsetHeight: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},
	contentRect: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},
	contentBoxSize: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},
	borderBoxSize: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},
	devicePixelContentBoxSize: {
	omit_in_ssr: true,
	invalid_elements: ['svelte:window', 'svelte:document']
	},

Describe the proposed solution

Add bindings for scrollWidth and scrollHeight for all elements.

Importance

nice to have

[nounder]

This would require contents tracking of target element via MutationObserver or through other svelte means.

Any cues or no-gos? I'd like to implement this feature.

[nmzein]

Hey is anything stopping this being a thing? I've encountered multiple times where this would be useful and less boilerplate than an event listener. I can try to help get this implemenented if its okay with everyone.