Add error visualization web app

Basic workflow:

- If a test or `Task` fails, it outputs a special URL like https://opensolid.github.io/error-visualization/1.0/index.html#8J+Ri/CfpJM=, where the fragment identifier is something like Base64-encoded JSON that describes a 2D or 3D visualization of the error
- The web app parses the fragment and displays the visualization (possibly allowing interaction via zoom/pan etc.)

There would then need to be a Haskell API for constructing error visualizations, and a corresponding Elm app that can display them.

Note that passing the data in the fragment identifier instead of a query parameter means that the error data never actually leaves the local machine - it's not part of the URL sent to the server, it's just handled by JS code running the local browser. This means that there shouldn't be any IP/privacy issues (as long as the web app itself doesn't do anything like send data off to a third party, of course - but it won't!).