Add syntax highlighting support for Stan

Summary: Add syntax highlighting support for the Stan probabilistic programming language (http://mc-stan.org/).

Reviewers: Framework: Syntax Highlighting, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, cullmann, dhaumann, pino, kde-frameworks-devel

Tags: Frameworks, Kate

Differential Revision: https://phabricator.kde.org/D13940