Do not allow workspace folders with scheme different than the current…

[isidorn]

Fixes https://github.com/microsoft/vscode-remote/issues/1608

I simply put the check in the doAddFolder of the worksapceEditingService. That if in the remote context we do not allow to add new folders which bring a new scheme to the story. The newly added folder must have a scheme which is already known.

Let me know what you think and I can change that.
Maybe we should make it more explicit that when this.environmentService.configuration.remoteAuthority there is already one folder

[sandy081]

• Does the operation should fail in this case? Otherwise there will be no feedback at all.
• What happens if user adds a folder by editing the file?

[isidorn]

1. Good point -> I should throw some error / notification so the user gets feedback
2. I hoped that also goes through this api, did not check. Will do tomorrow

[isidorn]

I have update the PR such that we show a warning when the user tries to drop a folder with a new scheme.
Here's the wording in case you have feedback on how to improve
"Workspace folders from different providers are not allowed in the same workspace."

As for the user editing the file that will be tackled with the explorer decoration provider and will not be a part of this PR.