Add data-marpit-svg attribute to SVG element outputted by inline SVG mode

[yhatt]

To be available selecting wrapped SVG by CSS query, we added data-marpit-svg attribute to SVG element outputted by inline SVG mode.

It would be used in marpit-svg-webkit-polyfill, to resolve the rendering bug in WebKit.

<svg data-marpit-svg="" viewBox="0 0 1280 960">
  <foreignObject width="1280" height="960">
    <section>
      ...
    </section>
  </foreignObject>
</svg>