Add GH action to generate .vsix artifact (#434)

* First try at GH action to generate `.vsix` artifact

* This thing is actually a zip file

* Fix GH SHA for artifact name

* Use latest `upload-artifact`

.github/workflows/ci.yaml

@@ -0,0 +1,39 @@
+
+name: Build VS Code Extension
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  workflow_dispatch:
+
+jobs:
+  package-extension:
+    runs-on: ubuntu-latest
+    outputs:
+      vsixPath: ${{ steps.package_extension.outputs.vsixPath }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: latest
+      - run: yarn install --immutable --immutable-cache --check-cache
+
+      - name: Package extension
+        uses: HaaLeo/publish-vscode-extension@v1
+        id: package_extension
+        with:
+          pat: not-a-real-pat
+          yarn: true
+          dryRun: true
+          preRelease: true
+          packagePath: apps/vscode
+
+      - name: Upload extension vsix file as a workflow artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: quarto-vscode-${{ github.sha }}
+          path: ${{ steps.package_extension.outputs.vsixPath }}