ci: add testing & shellcheck workflows (#159)

.github/workflows/shellcheck.yml

@@ -0,0 +1,33 @@
+---
+name: Shellcheck
+permissions:
+  contents: read
+on:
+  pull_request:
+    paths-ignore:
+      - "*.md"
+      - "assets/**"
+  push:
+    paths-ignore:
+      - "*.md"
+      - "assets/**"
+    branches:
+      - main
+  workflow_dispatch:
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@master
+        with:
+          severity: error
+          additional_files: "catppuccin.tmux"
+        env:
+          SHELLCHECK_OPTS: "-s bash"