Add theme.json path to index block template paths

Author: iftakharul-islam

src/wp-includes/class-wp-theme.php

@@ -1583,6 +1583,7 @@ public function is_block_theme() {
 		$paths_to_index_block_template = array(
 			$this->get_file_path( '/templates/index.html' ),
 			$this->get_file_path( '/block-templates/index.html' ),
+			$this->get_file_path( 'theme.json' ),
 		);
 
 		$this->block_theme = false;