export WebGLRenderingContext and add gl consts

[michaeljherrmann]

I was trying to use headless-gl to polyfill window.WebGLRenderingContext (in addition to canvas.getContext()). However, when working with just the class definition instead of a class instance, it was missing the gl consts which are available in the browser implementation. Eg:
WebGLRenderingContext.STENCIL_TEST

As a bonus, I also thought it would be nice to have WebGLRenderingContext as part of the top level exports instead of having to reach into gl/src/javascript/webgl-rendering-context.

I'm looking for feedback on this solution, as I'm not very familiar with this repo. But instead of just opening an issue, I thought I would try to get a start on some code. 🙂

[michaeljherrmann]

@dhritzkiv what do you think?

[dhritzkiv]

This looks good! Sorry I missed it the first time around, and thanks for bumping the thread.

I'll merge it shortly, and try to get a release out on npm

[michaeljherrmann]

Awesome, thank you! 🎉