Add global constant `YII_DEBUG_VERSION`

[vjik]

This will allow us to check for existing Yii Debug in app. Also we will can check Yii Debug version via compare_version().

Constant may be used in package configurations instead of params, for example here: https://github.com/yiisoft/yii-view-renderer/blob/master/config/events-web.php

Constant may be defined in PHP file that include via "autoload" section in Composer.