Support custom `/acf-json` locations

The acfe_folder_exists($folder) function is only used to check for the existence of the THEME + /acf-json folder to determine if the ACF->JSON sync is enabled. This function is a bit generic, but is only used in 2 places and both check for the same folder (./acf-json). The issue is that this path can be customized using the acf/settings/save_json filter, and in my case I would like to save them to WP_CONTENT_DIR + /acf-json to allow us to change themes and keep the same ACF settings.

This has been added to WP_CONTENT_DIR/mu-plugins/functions.php:

add_filter('acf/settings/save_json', function( $path ) {  
  return WP_CONTENT_DIR . '/acf-json';  
});

Instead of passing in the $path, using the acf_get_setting('save_json') function to get the path will run it through the filters and then will return is_dir() on the correct location. Perhaps this could be a different function if you intend to check for other folders in the theme at some point.

/**
 * JSON folder Exists
 */
function acfe_json_folder_exists(){

    $json_path = acf_get_setting('save_json');

    return is_dir($json_path);
    
}

As a side note, if you have your code somewhere I can submit a PR I would be happy to do so. Thanks!