Improve TypeScript support + JSDoc

[rgossiaux]

This branch contains some big updates to TypeScript support by using the $$Props feature. This will:

• Make TS understand the types of class and style correctly when used as functions of their slot props
• Understand all HTML attribute props and reject unknown props that do not exist on the element, the same as HTML elements, which will make it much easier to eg convert React Tailwind UI code
• Add JSDoc comments to props, which will be visible in your editor.

Merging this is blocked by a bug in SvelteKit's packaging: see sveltejs/kit#3783 I opened a PR for a possible fix to the language tools sveltejs/language-tools#1405 and depending on whether this or something else is merged, another (tiny) PR to Kit may be required.

[vercel]

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