allow prerendered pages to link to non-prerenderable endpoints

[Rich-Harris]

Fixes #5079. Most of the pieces were in place, but currently SvelteKit errors if you have a <link> in a prerendered page that points to a non-prerenderable +server.js file.

With this PR, if the crawler hits those routes, it won't error but nor will it prerender the route.

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Latest commit: d6628a7

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@sveltejs/kit	Patch

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