Definition: Expose command markers

Summary:
Added functios include:

• QString singleLineCommentMarker() const
• CommentPosition singleLineCommentPosition() const
• QPair<QString, QString> multiLineCommentMarker() const

Note: Some highlighting xml files define multiple singleLine and multiLine
comments. Currently, the last one wins...

Test Plan: make && make test

Reviewers: cullmann, vkrause

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D14763