Added ClojureScript and cljx extensions to clojure fileExtensions

[ClojureScript](https://github.com/clojure/clojurescript/) is a Clojure implementation targeting JavaScript. [cljx](https://github.com/lynaghk/cljx) helps write Clojure/ClojureScript portable projects.
Both are very common and should probably be recognized out-of-the-box by Brackets.