CoffeeScript: add extension .cson

Summary:
`cson` is a CoffeeScript-Object-Notation. It is a format for storing data, like JSON, and it is a subset of the CoffeeScript language.

* https://github.com/bevry/cson

Reviewers: dhaumann

Reviewed By: dhaumann

Subscribers: #framework_syntax_highlighting

Tags: #frameworks, #framework_syntax_highlighting

Differential Revision: https://phabricator.kde.org/D13565