breaking: add $props.bindable()

[dummdidumm]

closes #10768
closes #10711

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[dummdidumm]

yay or nay for throwing a compiler error in this case? The argument against it would be setters on a class, but I'm wondering how big of a deal it really is to make the prop bindable in that case, since this will in the vast majority of cases guide people towards the right outcome. Another argument for keeping it is that besides the runtime error telling you that you can't bind: to a prop when it's not declared with $props.bindable() is the only runtime error you get. When mutating a prop, you'll only get the "you better not mutate this" warning.

[dummdidumm]

Ugh .. if we keep the compiler error then we need to have a SvelteKit release veeery soon after it, because, as you can see, the site won't build right now since we're getting a compiler error for our generated root.svelte.