Add Svg.Attributes.classList

[sentience]

Just as in HTML, it’s useful to be able to create a space-separated list of classes to be applied to an SVG element. Html.Attributes.classList does this for HTML. For Svg, we must implement this ourselves.

Happy to submit a PR with an implementation if this would be considered helpful.

Current as of Elm 0.18.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.