.gitignore: Add CLion directories and VS build directory
Add IDE directories to .gitignore:
- CLion work and default build directories
- Visual Studio default build directory (used when opening the project via the "Open CMake" feature)
Notice: The VS build directory is simply called "out". Since no other path in the .gitignore is rooted (even the generic "Testing" entry) and there are no conflicts yet, I also added it non-rooted.