Draft: SVG output format for inlining into HTML

Towards #2285 (closed)

Also working around #2227 (closed)

$ echo 'digraph {}' | /usr/local/bin/dot -Tsvg_inline
<!-- Generated by graphviz version 6.0.2~dev.20221007.1037 (20221007.1037)
 -->
<!-- Pages: 1 -->
<svg width="8pt" height="8pt"
 viewBox="0.00 0.00 8.00 8.00">
<g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 4)">
<polygon fill="white" stroke="none" points="-4,4 -4,-4 4,-4 4,4 -4,4"/>
</g>
</svg>