samples: Add screentext.con as previously present under source/duke3d/src/lunatic

I've stumbled upon this Wiki page referencing the file which was removed in 2020 (along with the rest of the lunatic subdir), thus bringing the attention to it here: https://wiki.eduke32.com/wiki/Screentext

For reference, I'll link to other removed files, as you'll maybe find other files like screentext.con that you'll want to restore: https://voidpoint.io/terminx/eduke32/-/tree/dc0e6cc00a71235d788cc4a5c2e1d17de85a7ae0/source/duke3d/src/lunatic/test