Add common GLSL file extensions (.vs, .gs, .fs)

Summary:
These 2-letter extensions are used by, for example:

• https://www.interactiveshaderformat.com/about
• https://github.com/euler0/sublime-glsl
• https://github.com/polym0rph/GLSL.tmbundle
• http://plugins.netbeans.org/plugin/46515/glsl-syntax-highlighter

Test Plan: This commit also adds a test to ensure that .fs still defaults to FSharp.

Reviewers: Framework: Syntax Highlighting, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

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

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D22662