chore(gitignore): add compiled C++ and cache

Signed-off-by: Matej Focko <me@mfocko.xyz>

.gitignore

@@ -5,3 +5,10 @@ playground
 
 # ignore VSCode, cause screw it
 .vscode
+
+# ignore compiled C++ files with given names
+cpp/*
+!cpp/*.cpp
+
+# ignore cache directory
+.cache/