Fix end of folding region in rules with lookAhead=true

Summary:
The text length of end/begin of folding region is 0 if the respective rule has lookAhead="true".

For example:

Reviewers: Framework: Syntax Highlighting, cullmann, dhaumann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: andreasgr, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18516