github: Add shellcheck + shellfmt action.

Author: alexmv

.github/workflows/shell-test.yaml

@@ -0,0 +1,14 @@
+---
+name: Shell check and lint
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: luizm/action-sh-checker@master