Hotwire `CodeSpan` to diagram-elements

[lukaslueg]

It would be extremely useful to bidirectionally map span information to diagram elements so that

• we can highlight sections in the source-text which correspond to a diagram-element. The user might pick (mouseover) an element; the corresponding sections in the source should be highlighted; there may be multiple such sections in different arms (think of the "vertex" terminal in gfx_defines, which in the optimized form appears once but has two sources).
• we can highlight sections of the diagram which correspond to a syntax-element. The user might select parts of the text. The diagram-elements which are sourced by that part of the text should be highlighted.

This requires us to acquire, collect and map CodeSpan-information up to the point where we generate a diagram. The code to add custom data-attributes to an SVG-element is already in place. In addition to a diagram, we also give back some form of code-map. Some JS-code can then control the blinkenlights via CSS-rules regarding the affected SVG-nodes.