CI: Add GitHub action to check project formatting

Prettier is used to validate file formatting rules. CI job fails if files are not properly formatted.

.github/workflows/verify.yaml

@@ -0,0 +1,20 @@
+name: Verify project
+
+on:
+  pull_request:
+    branches: ["main"]
+  push:
+  workflow_dispatch:
+
+jobs:
+  verify:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out project
+        uses: actions/checkout@v3
+
+      - name: Check project formating
+        uses: creyD/[email protected]
+        with:
+          prettier_options: --check .