Include 'extensions.json' which describes recommended extensions for …

…development

Fixes #6005

- added the `.vscode/extensions.json` file which is used to describe recommended
extensions which are used for development. This will help collaborators install
extensions which are important for Theia's development and help in productivity.

Signed-off-by: Vincent Fugnitto <vincent.fugnitto@ericsson.com>

.vscode/extensions.json

@@ -0,0 +1,10 @@
+{
+  // See http://go.microsoft.com/fwlink/?LinkId=827846 to learn about workspace recommendations.
+  // Extension identifier format: ${publisher}.${name}. Example: vscode.csharp
+  // List of extensions which should be recommended for users of this workspace.
+  "recommendations": [
+    "ms-vscode.vscode-typescript-tslint-plugin"
+  ],
+  // List of extensions recommended by VS Code that should not be recommended for users of this workspace.
+  "unwantedRecommendations": []
+}