test: Packaging smoke test (#4)

Added packaging smoke test. This test fails e.g. for bugs like in
#3.

Author: teogeb

.github/workflows/ci.yml

@@ -21,5 +21,7 @@ jobs:
         run: npm ci
       - name: Run lint
         run: npm run lint
-      - name: Run tests
+      - name: Run unit tests
         run: npm run test
+      - name: Run packaging smoke test
+        run: npx @vscode/vsce package