VS Code: Add extension recommendation file

Adds VS Code extension recommendations for the nRF Connect SDK, that
will cause VS Code to prompt users to install the official nRF Connect
extension pack.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>

Author: trond-snekvik

.vscode/extensions.json

@@ -0,0 +1,5 @@
+{
+        "recommendations": [
+                "nordic-semiconductor.nrf-connect-extension-pack"
+        ]
+}