chore: consolidate prerender output gates into a helper

[Nic-Polumeyv]

Implements the helper suggested in #16507 (comment), targeting the prerender-dev-errors branch.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 53cd356:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/53cd356153d365d766b76c9d4be86b89b6b9a47d

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16508

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