ci: introduce linter

.github/workflows/linter.yml

@@ -0,0 +1,26 @@
+name: "Linter"
+
+on: [pull_request]
+jobs:
+  tests:
+    name: Linter
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+      with:
+        # We must fetch at least the immediate parents so that if this is
+        # a pull request then we can checkout the head.
+        fetch-depth: 2
+
+    # If this run was triggered by a pull request event, then checkout
+    # the head of the pull request instead of the merge commit.
+    - run: git checkout HEAD^2
+      if: ${{ github.event_name == 'pull_request' }}
+
+    - name: Install Dependencies
+      uses: php-actions/composer@v6
+      run: composer install --ignore-platform-reqs --no-cache
+
+    - name: Run Linter
+      run: ./vendor/bin/phpcs -p || true