Expand prerender crawling for improved DX

[jasongitmail]

Specifically, a few common situations that would benefit from it:

1. sitemap.xml - could be found by crawling robots.txt's Sitemap: <url> line.
2. blog/atom.xml or similar - could be found by crawling any link rel alternate's path -- e.g. <link rel="alternate" href="/blog/atom.xml" type="application/atom+xml" /> or <link rel="alternate" href="/blog/atom.xml" type="application/rss+xml" />
3. dynamic OG image routes -- could be found by crawling <meta property="og:image" content={ogImage} />

1 & 2 are easily to add to svelte.config.js via:

const config = {
	kit: {
		prerender: {
			entries: ['*', '/sitemap.xml', '/blog/atom.xml'],
		},
	},
}

But 3 is harder because the route depends upon a dynamic param where the title of the posts may be unknown beforehand.

But in any case, the main point is to suggest these as an opportunity to improve the DX so other devs don't bump into prerendering errors for these common situations.

I'll create a feature request if this seems reasonable.