Playground: Click on output item to highlight corresponding Rimu code

[ahdinosaur]

Is your feature request related to a problem? Please describe.
Now with #42 done, each output value has a span to the corresponding Rimu code.

Describe the solution you'd like
When you click on an output item, the corresponding Rimu code is highlighted.

Something like https://lezer-playground.vercel.app/

Describe alternatives you've considered

Additional context

• Will require being able to serialize and maintain a mapping from value spans to serialization spans. So custom serializers, not easy.