Registers the custom classname block attribute for block types that support it.

function wp_register_custom_classname_support( $block_type ) {
	$has_custom_classname_support = block_has_support( $block_type, array( 'customClassName' ), true );

	if ( $has_custom_classname_support ) {
		if ( ! $block_type->attributes ) {
			$block_type->attributes = array();
		}

		if ( ! array_key_exists( 'className', $block_type->attributes ) ) {
			$block_type->attributes['className'] = array(
				'type' => 'string',
			);
		}
	}
}