add support for extension-added functions and keywords

[brendankenny]

e.g. GL_OES_standard_derivatives's dFdx/dFdy give no matching overloaded function found errors.

This should be in ANGLE, we possibly just compiled the translator before some of these made it as standard extensions

[marquizzo]

If there were a file with a list of keywords we could manually add, that would be very helpful.

For example, Three.js, a WebGL library, has a bunch of baked-in variables that throw errors as the package exists today. Some common examples include position, modelViewMatrix and a bunch others.