Prerendering corner cases

[lilnasy]

Changes

• closes Prerended custom 500 page does not include imported stylesheets #8005
• maybe also Vercel adapter + prerender + build split leads to an error #7973
• Makes sure prerendered pages are turned into no-ops
• Uses prerendered 500 routes using the approach introduced in Refactor 404 and 500 approach #7754

Testing

• Adjusted node/prerender-404 to actually check that it prerendered
• Added a test that checks styles/scripts are included for indirectly routed 500 pages

Docs

Does not affect usage

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Latest commit: 7c17ee5

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[lilnasy]

Ready for review.

The change did not fix #7973 unfortunately.

[natemoo-re]

This looks great, thank you so much for digging into this!