Add solution for "Day 4: Printing Department", part 1

src/main.rs

@@ -1,3 +1,9 @@
+mod printing_department;
+
+use printing_department::PrintingDepartment;
+
 fn main() {
-    println!("Hello, world!");
+    println!("### Advent of Code 2025 ###");
+    let mut solver = PrintingDepartment::new();
+    solver.run();
 }