CI: Add hlint workflow

Closes: https://github.com/xmonad/xmonad-contrib/issues/669

.github/workflows/hlint.yaml

@@ -0,0 +1,22 @@
+name: hlint
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  hlint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: 'Set up HLint'
+      uses: haskell/actions/hlint-setup@v2
+      with:
+        version: '3.4.1'
+
+    - name: 'Run HLint'
+      uses: haskell/actions/hlint-run@v2
+      with:
+        path: '["XMonad/", "tests/", "scripts/"]'
+        fail-on: status