Add syntax definition for Adblock Plus filter lists

Summary:
This will allow us to replace the handwritten highlighter in the KDE PIM
adblocker plugin.

Reviewers: #framework_syntax_hightlighting, mlaurent, dhaumann

Reviewed By: #framework_syntax_hightlighting, dhaumann

Subscribers: dhaumann, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7661