Add a workflow to auto-label new PRs with team:viewer.

When a PR appears on the QA board, QA wants to know whose team it came from.
So every PR on the viewer repo should be labeled with team:viewer.

.github/workflows/label_pr.yaml

@@ -0,0 +1,17 @@
+name: Label PR with team:viewer when first opened
+on:
+  pull_request:
+    types:
+      - opened
+jobs:
+  label_pr:
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+    steps:
+      - run: gh pr edit "$NUMBER" --add-label "$LABELS"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_REPO: ${{ github.repository }}
+          NUMBER: ${{ github.event.pull_request.number }}
+          LABELS: "team:viewer"