powered_by_link not providing an accessible warning

[ludoboludo]

The powered_by_link object doesn't output a link that is deemed accessible enough.

The link is being opened in a new window/tab so having a warning for screen readers can be useful. We tend to add this behaviour on our Shopify flagship themes.

Here is the github issue from our public repo outlining the issue: https://github.com/Shopify/dawn/issues/720

We could create work around via JS but ideally we don't need to add extra logic like this to make the outcome of the object accessible.

[ludoboludo]

Also mentioned in this PR: https://github.com/Shopify/dawn/pull/1735