insert http-equiv cache-control tag when prerendering

[Rich-Harris]

If a page specifies a maxage in its load result, a cache-control header is generated. Since apps don't (typically) have any control over headers with prerendered content, this is the next best thing. Mentioned briefly in #909 (comment).

This also lays the groundwork for inserting CSP headers in prerendered content.

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🦋 Changeset detected

Latest commit: c17662f

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@sveltejs/kit	Patch

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[benmccann]

seems reasonable to me