Add CS workflow

.github/workflows/cs.yml

@@ -0,0 +1,51 @@
+# yamllint disable rule:line-length
+
+name: Code Style
+
+on:
+  pull_request:
+  push:
+    branches:
+      - trunk
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        php-version: ['7.4']
+
+    name: PHPCS
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          coverage: none
+          php-version: ${{ matrix.php-version }}
+          tools: composer:v2
+
+      - name: Get Composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-dir)"
+
+      - name: Cache dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: composer-${{ runner.os }}-${{ matrix.php-version }}-${{ hashFiles('composer.*') }}
+          restore-keys: |
+            composer-${{ runner.os }}-${{ matrix.php-version }}-
+            composer-${{ runner.os }}-
+            composer-
+
+      - name: Install dependencies
+        run: |
+          composer install --prefer-dist --no-interaction --no-progress
+
+      - name: Check code style
+        run: |
+          composer php:lint