Playground: ability to render specific target

[timbertson]

I'm new to D2 (looks great!) and am eager to try out the layer/scenario compositions.

It looks like the playground can only show the base layer, which is a shame. Setting up the tooling and using the CLI is a bunch of friction when you just want to try something out interactively.

The simplest option would be to add a Target dropdown to the UI which ends up specifying the --target CLI argument. A more advanced option would be to support animated SVG etc in the playground, though TBH the Target dropdown might be better for most use cases.