Add warning level to shellcheck

Signed-off-by: Judy Ng <[email protected]>

.github/workflows/shellcheck.yml

@@ -0,0 +1,15 @@
+name: Shellcheck
+
+on: [push, pull_request]
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@main
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@master
+        with:
+          severity: warning
+          ignore_paths: cookbooks/third-party