fix: prerender errors during development

[teemingc]

We've been tracking the current page's prerender option during SSR so we might as well enable some page prerendering behaviours during development to match what they'll get during the build.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 8ef31c6:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/8ef31c60382b3ccd9eca70f5a0599938b642cf0c

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16507

[changeset-bot]

🦋 Changeset detected

Latest commit: 8ef31c6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16507-svelte.vercel.app/

[Nic-Polumeyv]

state.prerendering || state.prerender_default === true appears nine times now, could be one helper:

/** @param {import('types').SSRState} state */
export function renders_prerendered_output(state) {
	return !!state.prerendering || (DEV && state.prerender_default === true);
}

The DEV guard is optional. Prerendered routes are filtered out of the server manifest at build time so these gates can't fire in production anyway, but that invariant lives in the adapter layer, and this makes the changeset's "during development" true by construction.