follow redirects when prerendering

[Rich-Harris]

When prerendering, if a page or endpoint redirects to another location, the redirect is logged but not followed. In a case I encountered just now, a /docs link in the nav bar was followed, but the page consisted of this...

<script context="module">
  /** @type {import('@sveltejs/kit').Load} */
  export function load() {
    return {
      status: 308,
      redirect: '/docs/getting-started'
    };
  }
</script>

...and /docs/getting-started was never crawled. Easily fixed by ensuring that there's a link to the correct URL, but I would expect Kit to do the right thing here regardless.

Haven't added a test yet as I'm under the cosh

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[benmccann]

we've had bugs with this logic before and it's in a few different places like line 129 of load_node. I wonder if we should make a util function for it. I think this will need to be updated to match load_node (and I'm pretty sure it's somewhere else too - maybe elsewhere in prerendering):

resolved.startsWith('/') && !resolved.startsWith('//')

[Rich-Harris]

good catch, added a util. bonus — it's significantly faster than calling startsWith

[benmccann]

I noticed one issue, but looks good to me other than that