Add GLSL syntax highlighting support

[zesterer]

I suggest it be applied to files with the extensions .glsl, .vert and .frag.

For now, the syntax is similar enough to C that using C syntax highlighting is probably adequate for an initial implementation.

I'm happy to contribute any time I can spare towards this.

[refi64]

You could probably just go to bundles/c/init.moon and add the extensions to the extensions: table.