svg: manually advance through animation

[timbertson]

I'm trying to make some animations with d2 to illustrate algorithms. It's nice to animate these, but using a specific animation-interval means people can easily get lost if the animation goes too fast.

With pdf/pptx you allow the user to advance at their own pace, but embedding PDFs comes with a lot of visual bulk (toolbar and sidebar).

It seems like it should be possible to embed forward/backward controls in SVG, I hacked up a POC in a branch here. You can see the rendered result here. Obviously styling can be improved, this is just to show that it's doable.

Would you be interested if I polished this up and opened a PR, adding it as an (optional) alternative to --animation-interval when exporting to SVG?

I'd also love to get keyboard arrow (+space) shortcuts working as a stretch goal, I think that should be possible as well.