Improve Solid Security's global variable definition handling

Ensure Solid Security checks if a WP global variable is already defined before setting it in wp-config.php, preventing potential conflicts like WSOD with other plugins or custom code.

For example, instead of directly adding without checking:

// BEGIN iThemes Security - Do not modify or remove this line
// iThemes Security Config Details: 2
define( 'DISALLOW_FILE_EDIT', true ); // Disable File Editor - Security > Settings > WordPress Tweaks > File Editor
// END iThemes Security - Do not modify or remove this line

Configure it to:

if(!defined('DISALLOW_FILE_EDIT')) {
	define('DISALLOW_FILE_EDIT', env('DISALLOW_FILE_EDIT', true));
}

Request: https://wordpress.org/support/topic/definewsod-issue/