Add PHPStan to CI.

Author: Crell

.github/workflows/quality-assurance.yaml

@@ -23,3 +23,18 @@ jobs:
                   tools: composer:v2
             - run: composer install --no-progress ${{ matrix.composer-flags }}
             - run: vendor/bin/phpunit ${{ matrix.phpunit-flags }}
+    phpstan:
+        name: PHPStan checks on ${{ matrix.php }}
+        runs-on: ubuntu-latest
+        strategy:
+            matrix:
+                php: [ '7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5' ]
+        steps:
+            - uses: actions/checkout@v2
+            - uses: shivammathur/setup-php@v2
+              with:
+                  php-version: ${{ matrix.php }}
+                  coverage: xdebug
+                  tools: composer:v2
+            - run: composer install --no-progress
+            - run: vendor/bin/phpstan