LaTeX: fix folding in \end{...} and in regions markers BEGIN-END

Summary:
BUG: 419125

Fix folding in \end{...}

In \end{...} the folding region "block" is terminated twice, which causes a folding error.

Add folding in regions BEGIN/END

Add code folding in the rules of the region markers BEGIN/END. The context "FindComments" is created with the rules of comments.

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D29521