Add PHP 8 to code checks

Author: andrewdalpino

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         operating-system: [windows-latest, ubuntu-latest, macos-latest]
-        php-versions: ['7.4']
+        php-versions: ['7.4', '8.0']
 
     steps:
       - name: Checkout