Add a Github Actions workflow

[theoparis]

This just builds each .cpp file in a subfolder with the clang compiler and then stores the compiled program in a artifact on Github. The last step isn't really necessary here though. I enabled most of the important warnings from the compiler, but did not add -Werror so it shouldn't cause any major compilation errors (just warnings).

I should mention that it builds the code into a single welcome.o file for example, and does not place it into a subfolder in the out folder. So if there are two files with the same name, in different weeks, it might cause problems atm. I'm yet to figure out how to improve the github action so that it adds a week_3_welcome_ prefix.