Add remaining alternative gdi04 missions

This adds gdi04b (scg04wa) and gdi04c (scg04wb).

gdi04b is similar to gdi04a objective-wise, gdi04c is completely different. Both have been tested with the latest balance changes. gdi04b is of medium difficulty, I'd say, and gdi0c is kinda hard. You probably need a few tries to figure out how to get past the start and survive with enough units to reach the end.

I did not want to get these merged while old lua still had the deprecation notice, but now that that is removed, I'd like to see them in. They are very much self-contained, so shouldn't cause any issues.