Clean up initial extension installation on vscode web

[jeanp413]

Now that theia integration is being removed #7395, we should do some cleanup when we do the initial extension installation in vscode web.
Right now, we have some custom logic inside gp-code branch to do the extension installation, we need to simplify and move this logic to the supervisor and install the needed extension using the server argument --install-entension and install-builtin-extension

[akosyakov]

Supervisor should be kept agnostic to concrete IDEs. We have though startup.sh and we could move this logic there.