extract public to schema variable so we can use a constant later

pg4wp/rewriters/ShowTablesSQLRewriter.php

@@ -4,6 +4,7 @@ class ShowTablesSQLRewriter extends AbstractSQLRewriter
 {
     public function rewrite(): string
     {
-        return 'SELECT tablename FROM pg_tables WHERE schemaname = \'public\';';
+        $schema = "public";
+        return 'SELECT tablename FROM pg_tables WHERE schemaname = \'$schema\';';
     }
 }