fix: add "false" value to preload link options

[teemingc]

closes #10460

Explanation: #10460 (comment)

We should update the language tools as well. I think the original PR from @alesvaupotic originates from the extension incorrectly stating that "false" is not a valid value (although it is for the non-preloading link options).

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[dummdidumm]

The other link options support false since #10039, though the preload link options were seemingly deliberately left out - I'm not sure if this just slipped through or was a conscious decision. Maybe @Rich-Harris remembers.

[dummdidumm]

We can merge this into SvelteKit 1 (I'm in favor) but we can't change the types to include "false" until SvelteKit 3 because of potentially conflicting type definitions (SvelteKit 2 will have these definitions under its own control, but it supports Svelte 4 which also provides these, and so we can't have conflicts in there)