chore(clish): check gd extension

Author: adhocore

bin/clish

@@ -13,6 +13,12 @@ if (\Phar::running() && !\extension_loaded('zlib')) {
     exit(1);
 }
 
+if (!\extension_loaded('gd')) {
+    echo "clish requires gd extension for exporting highlighted code to png image\n";
+
+    exit(1);
+}
+
 if (\file_exists(__DIR__ . '/../../../autoload.php')) {
     require __DIR__ . '/../../../autoload.php';
 } elseif (\file_exists(__DIR__ . '/../../autoload.php')) {