ci: Add hlint workflow

The "haskell/actions/hlint-run" GitHub Action produces annotations which
GitHub shows in the code diff, in addition to just failing the workflow
when there are any suggestions. Also, now it runs in parallel to the
other workflows.

Related: https://github.com/xmonad/xmonad-contrib/issues/669
Related: dd26fcc3f1

.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: '.'
+        fail-on: status