Add support for github.dev

[deemp]

CodeMirror is OK for sharing snippets.

What do you think about browser-based development without installing the rzk binary?

https://docs.github.com/en/codespaces/the-githubdev-web-based-editor

For the rzk playground, we build a js script that registers the rzkTypecheck_ callback. The main TS application uses this callback for typechecking. Communication happens via a mutable object. More callbacks can be added if necessary.

[aabounegm]

That is a great idea and I've been thinking about it already. I believe to support github.dev, we need the extension to be compatible with Web Extensions. I'm not yet sure how easy that would be to achieve, since it probably means compiling the LSP implementation to JavaScript which doesn't seem to be an easy task