Add index.php to prevent server errors on plugin directory URL

Fixes 5xx errors reported in Google Search Console for
/wp-content/plugins/code-syntax-block/ — empty index.php
returns a blank 200 instead of a server error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Andrew Baker

index.php

@@ -0,0 +1 @@
+<?php // Silence is golden.