Anchor should have rel="noopener noreferrer" when target="_blank"

[halocline]

Please describe what the rule should do:

Anchor should have rel="noopener noreferrer" when target="_blank"

What category of rule is this? (place an "X" next to just one item)

[ ] Warns about a potential error (problem)
[ ] Suggests an alternate way of doing something (suggestion)
[x] Other (please specify:) a current best practice for security

Provide 2-3 code examples that this rule will warn about:

<Anchor target="_blank" href="https://my.domain.com" label="Whisk me away" />

Why should this rule be included in eslint-plugin-grommet?

Provides warning suggesting best practice. If accompanied by a quick fix, allows for automation.