Fully decouple `./webview` from VS Code

[jviotti]

Right we have ./ui/src/vscode-api.ts that assumes VS Code for communicating outside the web view. Maybe that can be called "channel" or "messaging" something like that and only internally detect VS Code, but others in the future.

Apart from that, ./ui seems to use a lot of VS Code specific CSS variables everywhere. I wonder how we can decouple that. Maybe we create our own variable names that map into VS Code for VS Code builds but we allow configuring them some other way too?

[jviotti]

Maybe it all happens on the index.css file. i.e. we abstract away the variables there so in other builds we provide a different CSS file? Maybe then the CSS file needs to live in ./vscode?

[jviotti]

Also, if we better abstract away ./ui/src/vscode-api.ts, then we can potentially test the UI in isolation, outside of VS Code too, by mocking input/output