Update `data-sveltekit-` attribute types to allow `boolean | "true" | "false"`

[Rich-Harris]

Describe the problem

See sveltejs/kit#10039 for context

Describe the proposed solution

We need to be more permissive here:

svelte/elements/index.d.ts

Lines 554 to 560 in 3bc791b

	// SvelteKit
	'data-sveltekit-keepfocus'?: true | '' | 'off' | undefined | null;
	'data-sveltekit-noscroll'?: true | '' | 'off' | undefined | null;
	'data-sveltekit-preload-code'?: true | '' | 'eager' | 'viewport' | 'hover' | 'tap' | 'off' | undefined | null;
	'data-sveltekit-preload-data'?: true | '' | 'hover' | 'tap' | 'off' | undefined | null;
	'data-sveltekit-reload'?: true | '' | 'off' | undefined | null;
	'data-sveltekit-replacestate'?: true | '' | 'off' | undefined | null;

After making this change, we should deprecate the use of off in SvelteKit.

(Open question: do we remove 'off' from the types here immediately, or leave it for now?

Alternatives considered

n/a

Importance

nice to have