Highlight CUDA .cu and .cuh files as C++

Summary: This patch makes the syntax highlighting use the C++ style for CUDA sources (.cu) and headers (.cuh) by default. This should work very well, since those are essentially C++ files with very little additional syntax.

Test Plan: Shortly tested manually in KDevelop, looked good.

Reviewers: Framework: Syntax Highlighting, dhaumann

Reviewed By: Framework: Syntax Highlighting, dhaumann

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

Tags: Framework: Syntax Highlighting, Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D17904