Added GLSL parser

[decioferreira]

An initial implementation for the WebGL Shader (GLSL) parser.

[Janiczek]

Looks good so far! Please let's add one more example to ParserTest with a correct GLSL program (can be very simple but probably should use the varyings etc. that change the type of the shader).

I'm curious to see how the shader type turns out! Don't hesitate to ask on Discord if you want help with generating the type equations :)

(Type inference tests will be the most interesting and important part here I believe.)

[decioferreira]

added two more examples to the shader parser tests, based of https://github.com/elm-explorations/webgl/blob/81527c23ba21d9bf92f34151a6a4c40a68460e1f/examples/Triangle.elm#L77-L105