fix #269 Download vscode.proposed.d.ts during build

Author: gjsjohnmurray

.github/workflows/main.yml

@@ -54,6 +54,7 @@ jobs:
       with:
         node-version: 12.x
     - run: npm install
+    - run: npx vscode-dts dev
     - name: lint
       if: runner.os == 'Linux'
       run: npm run lint