GH Actions: set error reporting to E_ALL

The default setting for `error_reporting` used by the SetupPHP action is `error_reporting=E_ALL & ~E_DEPRECATED & ~E_STRICT` and `display_errors` is set to `Off`.

For the purposes of CI, I'd recommend running with `E_ALL` and `display_errors=On` to ensure **all** PHP notices are shown.

Ref: shivammathur/setup-php#469

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         with:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl
-          ini-values: max_execution_time=600, memory_limit=256M
+          ini-values: max_execution_time=600, memory_limit=256M, error_reporting=-1, display_errors=On
           coverage: none
 
       - name: Use Composer 1.x
@@ -47,6 +47,7 @@ jobs:
         with:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl, sodium
+          ini-values: error_reporting=-1, display_errors=On
           coverage: none
 
       - name: Install Composer dependencies