Improve WASM demo

[matklad]

We have a WASM demo at https://matklad.github.io/tom/ (the code is on the gh-pages branch). It would be cool to add more features to it:

• Use TomlDoc for syntax and error highlighting (use monaco editor with language=null for this?)
• Synchronize selection in the doc on the left and tree on the right. Selecting node on the right should highlight region in the doc. Moving cursor in the doc should highlight the current node in the tree.
• Add "reformat buttom" (see Formatter #15 )
• Make the site look less like a Soviet brutalist building
• Add a third pane which shows equivalent JSON (see Add logical model #13)