Expose vscode remote reconnecting UI.

<!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Can an API be exposed to extensions to re-use vscode "Reconnecting" UI? Outside of vscode remote scenario, some extensions like liveshare do communication with another instance of vscode over the network. Being able to reconnect due to spotty network connection via the extension is very similar to the vscode remote scenario. Blocking the UI here, like vscode does during reconnection makes the network reconnection problem for liveshare simpler, else we would have to deal with coediting and other editor operations being out of sync. It would be nice, if extensions like liveshare can use the reconnecting UI that vscode uses for its remote scenario. cc @lostintangent