.frag file extension for JavaScript

This PR adds the .frag file extension for JavaScript as discussed in #1176.
GLSL also has this extension so I added a few samples for both languages.

Examples of JavaScript files misclassified as GLSL can be seen here:
https://github.com/search?q=return+extension%3Afrag&type=Code&ref=searchresults

It fixes the problem for the two repositories mentioned in #1176 ([katasemeion](https://github.com/Kazark/katasemeion) and [conway-game-of-life-with-angular-js](https://github.com/Kazark/conway-game-of-life-with-angular-js)).