Add Linter to GitHub Actions

.github/workflows/linter.yml

@@ -0,0 +1,23 @@
+name: Linter
+on:
+  push:
+    paths:
+      - '.github/workflows/linter.yml'
+      - '**.sh'
+  pull_request:
+    paths:
+      - '.github/workflows/linter.yml'
+      - '**.sh'
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@master
+        env:
+          SHELLCHECK_OPTS: -x
+        with:
+          ignore_paths: ./test/bats/** ./test/test_helper/**