fix: prerender no longer silently ignores webpages pertinent to #15620

[aenriii]

closes #15620
closes #10735

This PR addresses #15620, and adds tests for any possible regressions as a result of the implemented fix. (edit: seems to also fix #10735)

Changes:

• modified kit/src/core/postbuild/analyse.js to add additional metadata fields to route metadata, page.prerender and api.prerender, which pass on the page and endpoint prerender fields. this prevents a regression which forces all parts of a route to prerender even if only one part requests it.
• modified kit/src/core/postbuild/prerender.js to check if page.prerender and api.prerender is true, and enqueue prerendering if so. uses \x00page as a postfix to page keys within the preexisting seen set to differentiate between api endpoints and pages.
• modified kit/src/core/postbuild/prerender.js, added and documented expect_html parameter in visit() and enqueue() to specify that the Accept: text/html header should be sent to the internal server when prerendering a page.
• implemented two tests in prerendering/basics, one which tests the fix itself (see prerender-with-server-endpoint) and one which confirms the non-existence of a possible regression (see linked-api). both tests pass with flying colors :D

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Preview: https://svelte-dev-git-preview-kit-15621-svelte.vercel.app/

[aenriii]

regressions fixed, should also close #10735