ci: add stylua GitHub action (#20)

Closes #7.

.github/workflows/pr-checks.yml

@@ -0,0 +1,16 @@
+name: PR checks
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  stylua:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: JohnnyMorganz/stylua-action@v2
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          version: latest
+          args: --check .