Add ext-encoding to test workflow

In theory this should work?

Author: dktapps

.github/workflows/main.yml

@@ -20,6 +20,7 @@ jobs:
         uses: shivammathur/setup-php@2.32.0
         with:
           php-version: ${{ matrix.php }}
+          extensions: pmmp/ext-encoding@0.5.0
 
       - name: Cache Composer packages
         id: composer-cache