Modify .gitignore to track directories without source files

Author: yiwiy9

.gitignore

@@ -1,6 +1,7 @@
 # Generated by Cargo
 # will have compiled files and executables
-/target/
+debug/
+target/
 
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
 # More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
@@ -9,10 +10,15 @@ Cargo.lock
 # These are backup files generated by rustfmt
 **/*.rs.bk
 
+# MSVC Windows builds of rustc generate these, which store debugging information
+*.pdb
+
 # Ignore user settings file
 .env
 /.vscode/
 
-# Ignore source code files
+# Ignore source code files but track the directories themselves using .gitkeep
 /src/contest/*
+!/src/contest/.gitkeep
 /src/lib/src/*
+!/src/lib/src/.gitkeep