Add referrerpolicy="noreferrer" on built index

[leolb-aphp]

What problem does this feature solve?

In this issue: https://github.com/rancher/dashboard/issues/575 - they are importing external resources on what's usually deployed on internal infrastructure where leaking referrer is not a big deal but certainly unnecessary and a nuisance.

What does the proposed changes look like?

Allow one to specify either abitrary attributes on the stylesheet and script elements or just set the referrerpolicy for these.

So that one can do, e.g.

./node_modules/.bin/nuxt build --referrerpolicy "noreferrer"

_{This feature request is available on Nuxt community (#c10590)}