Add phpstan to CI.

Author: hbgl

.github/workflows/ci.yml

@@ -29,6 +29,7 @@ jobs:
       - name: Install dependencies
         run: |
           composer install --no-progress --prefer-dist --optimize-autoloader
-          composer require --dev phpunit/phpunit
+      - name: Check with phpstan
+        run: vendor/bin/phpstan analyse src --level 8
       - name: Test with phpunit
         run: vendor/bin/phpunit --coverage-text