add GitHub workflow

.github/workflows/check.yaml

@@ -0,0 +1,19 @@
+name: Check
+
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    branches:
+      - main
+jobs:
+  check:
+    runs-on: macos-latest
+    # TODO Xcode versions (multiple?)
+    steps:
+      - uses: actions/checkout@v4
+      - run: npm ci
+      - run: script/format/check
+      - run: script/lint/check
+      - run: swift build
+      - run: swift test