A11y: recognizing svg a links too

[paulocoghi]

When using SVG elements, we need to use xlink:href attribute in anchors for greater compatibility (Safari doesn't work with just href):

<svg>
    <text><a xlink:href="/just_a_url">Just a text</a></text>
<svg>

But in this case, A11y inform this warning that isn't correct (since there is an anchor):

A11y: <a> element should have an href attribute

With time, I will try to submit a pull request, if you want :)

[Rich-Harris]

A PR would be very welcome, thank you!