Add an air GHA now that I've completed The Great Reformatting

This should help to maintain consistent formatting going forward.

Author: jennybc

.github/workflows/format-check.yaml

@@ -0,0 +1,21 @@
+# Workflow derived from https://github.com/posit-dev/air-action/tree/main/examples
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+
+name: format-check.yaml
+
+permissions: read-all
+
+jobs:
+  format-check:
+    name: format-check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Format check
+        uses: posit-dev/air-action@v1
+        with:
+          args: format . --check