Extend preprocessor parsing

[KoltesDigital]

Add support for:

• else

• elseif

• endif

• error

• if

• ifdef

• ifndef

• include

• line

• pragma

• undef

[hadronized]

This is going to be very useful, especially for https://github.com/rust-gamedev/wg/issues/23, since we’re going to require to support the preprocessor at some point.

I’ll update the changelog and release as glsl-3.0.0 along with some other changes that occurred today and yesterday. Thanks a lot @KoltesDigital! :)