dns-prefetch links should be excluded

[wackget]

Links of type dns-prefetch should not be checked by lychee. This is because they are only used to perform DNS lookups and never to fetch any content, e.g.:

<link rel='dns-prefetch' href='https://fonts.googleapis.com/' />

I am aware of an existing issue #897 which covers preconnect-type links but dns-prefetch is a separate type not covered by the proposed fix.