Use a prelaunch task to build demo on debug

.vscode/launch.json

@@ -7,7 +7,8 @@
       "type": "extensionHost",
       "request": "launch",
       "runtimeExecutable": "${execPath}",
-      "args": ["--extensionDevelopmentPath=${workspaceRoot}/demo" ]
+      "args": ["--extensionDevelopmentPath=${workspaceRoot}/demo" ],
+      "preLaunchTask": "build"
     }
   ]
 }

.vscode/tasks.json

@@ -0,0 +1,14 @@
+{
+  "version": "0.1.0",
+  "command": "npm",
+  "isShellCommand": true,
+  "showOutput": "always",
+  "suppressTaskName": true,
+  "tasks": [
+    {
+      "isBuildCommand": true,
+      "taskName": "build",
+      "args": ["start"]
+    }
+  ]
+}