Add Marek Lewandowski as a CKEditor module maintainer

https://www.drupal.org/u/mlewand is currently the CKEditor 4 maintainer/lead developer, the successor of https://www.drupal.org/u/reinmar.

Effectively, the CKEditor team has written the vast majority of Drupal 8's custom CKEditor plugins: the drupallink, drupalimage and drupalimagecaption plugins. This is better for everybody, because it means less integration problems with other CKEditor plugins, and optimal usage of their APIs. We (Drupal), and specifically I (Wim Leers) have been making sure these Drupal-specific CKEditor plugins integrate with Drupal as well as possible. But without them, they would not be the same.

Furthermore, it has already happened quite a few times that a Drupal contributor reported a previously unknown CKEditor bug. That then needed an upstream bugfix.

So, effectively, the division of labor has been:

• All CKEditor issues are triaged by me.
• Anything PHP-related for the CKEditor module is handled by me.
• Anything JS-related in the CKEditor module that is independent from CKEditor the JS library is handled by me.
• Anything JS-related in the CKEditor module that is related to CKEditor the JS library is handled by the CKEditor team: I escalate this to them.

Therefore, I think it's valuable to update MAINTAINERS.txt to reflect this reality.