I don't know what could be more streamlined than clicking "Code -> Open in Codespace" on GitHub, and Visual Studio Code renders in your browser, with all of your dotfiles set and dependencies loaded to mess around with something.

(GitPod does a similar thing where you can append the URL to gitpod.io, but they can't use the VSCode extension marketplace).

Codespaces is in private beta. I have requested access for some time now and got no response.

All the services I mentioned are fully open for everybody today.

Unless, unimaginably, you need to keep your code on-prem...

I know, I know. Unthinkable.

I haven't tested, but I think gitpod works with on-prem installations.

Yes, Gitpod is open-source and can be self-hosted: https://github.com/gitpod-io/gitpod/