Build errors detected wrt preLaunchTask 'npm'.

[pascalpoizat]

Hello

I followed the guidelines here: https://typefox.io/building-a-vs-code-extension-with-xtext-and-the-language-server-protocol.
Upon pressing F5 I have an error: build errors have been detected during the 'npm' preLaunchTask.
("Des erreurs de build ont été détectées durant le preLaunchTask 'npm'.").
But for this the plugin works ok on the given example.

The errors I find in the problems tab are:

• the vscode module has no exported member 'WorkspaceFolder'.
• the vscode module has no exported member 'WorkspaceFoldersChangeEvent'.

VS Code Version 1.18.1 (1.18.1)
Mac OS 10.13.1 (17B1003)