Is there any support of defining extensions to use per repo?

Is there any support of defining extensions to use per repo?
Like the `.vscode/extensions.json` and the `recommendations` part?

I showed this by changing the url of a repo for someone, changing **github** to **github1s**, and the reaction was the same as for me, really cool, but he wanted the `go to implementation` in `c#`-interface-methods, and I guess that's only adding an `c#`-extension, but does those things work on this online version of vscode?