Add support got GLSL files

[joewood]

Related to #1517. Right now the only option for using GLSL shader files is to expose the shader as a string. Many editors provide good extension support for writing shaders. Using a Javascript string means missing out on syntax coloring and other analysis by the editor.

It would be much better to add default support for GLSL files via the raw-loader.

[Bjvanminnen]

Agree that this would be useful. In the meantime, I've managed to work around the issue by telling my editor to treat the file as if it were glsl (despite technically being js). I suspect this is an option in many editors, but in atom and vs code I was able to do so by clicking on the language in the lower right of the window. Hope this helps :)

[Timer]

Merging with #1944.

[gaearon]

See also: #3722.