Protect development mode constant from being re-defined (#14165)

When defining `GUTENBERG_DEVELOPMENT_MODE` in `wp-config.php` and running Gutenberg `master` directly from the repository, we'd see PHP warnings:

```
[28-Feb-2019 11:39:14 UTC] PHP Notice:  Constant GUTENBERG_DEVELOPMENT_MODE already defined in /var/www/html/wp-content/plugins/gutenberg/gutenberg.php on line 13
[28-Feb-2019 11:39:14 UTC] PHP Stack trace:
[28-Feb-2019 11:39:14 UTC] PHP   1. {main}() /var/www/html/wp-cron.php:0
[28-Feb-2019 11:39:14 UTC] PHP   2. require_once() /var/www/html/wp-cron.php:39
[28-Feb-2019 11:39:14 UTC] PHP   3. require_once() /var/www/html/wp-load.php:37
[28-Feb-2019 11:39:14 UTC] PHP   4. require_once() /var/www/html/wp-config.php:89
[28-Feb-2019 11:39:14 UTC] PHP   5. include_once() /var/www/html/wp-settings.php:342
[28-Feb-2019 11:39:14 UTC] PHP   6. define() /var/www/html/wp-content/plugins/gutenberg/gutenberg.php:13
```

gutenberg.php

@@ -11,7 +11,7 @@
  */
 
 ### BEGIN AUTO-GENERATED DEFINES
-define( 'GUTENBERG_DEVELOPMENT_MODE', true );
+defined( 'GUTENBERG_DEVELOPMENT_MODE' ) or define( 'GUTENBERG_DEVELOPMENT_MODE', true );
 ### END AUTO-GENERATED DEFINES
 
 gutenberg_pre_init();