Initial support for tooltips: parse tree visualizations.

[dan-zheng]

Dex has a reactive notebook and literate programming environment in dex web.

Tooltips add additional IDE information to the web environment, visible when hovering the cursor over code.

This initial PR adds infrastructure for tooltips, with parse tree visualizations as the first feature.

• Backend: compiler computes a SpanTree describing all nested expression spans, based on concrete syntax trees.
• Frontend: HTML rendering uses SpanTree to augment HTML elements with class attributes about span ranges. These span range attributes are highlighted on mouse hover.

First step towards https://github.com/google-research/dex-lang/issues/1304.

Demo

dex web

dex --prelude /dev/null web lib/prelude.dx 2>&/dev/null

Dex tooltips demo video

Exported HTML

Created via make docs.

prelude.html behaves exactly the same as the live demo above.

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[dan-zheng]

I think this PR is ready for review. I spent some time this week cleaning the code.

tests/unit/SourceInfoSpec.hs could be cleaned up and could have more testcases.

I'm happy to take a look later this week, or in a follow-up PR.