add support for multiple rel values when checking for external links

[lukasIO]

This PR proposes two changes:

1. Add "noopener" and "noreferrer" to the checklist in router.js in order to bail on navigation if one of these is present
2. In the process I found that the spec says [1] that the rel attribute should be parsed as a list of values that are seperated by a white space. So this PR tries to address that. Which in turn makes it possible to write stuff like rel="external nofollow" without breaking the SvelteKit navigation check

[1] https://html.spec.whatwg.org/#linkTypes

To determine which link types apply to a link, a, area, or form element, the element's rel attribute must be split on ASCII whitespace. The resulting tokens are the keywords for the link types that apply to that element.

[lukasIO]

not sure why the tests are failing in the CI, they pass locally (running Ubuntu 20.04)

[benmccann]

Please add a changeset in the future. I did it for you on this one