feat: add exit builtin command

src/builtins/exit.rs

@@ -0,0 +1,5 @@
+use crate::error::ShellError;
+
+pub fn run() -> Result<(), ShellError> {
+    Err(ShellError::Exit)
+}

src/builtins/mod.rs

@@ -1,7 +1,8 @@
 use crate::error::ShellError;
 mod cd;
+mod exit;
 
-const BUILTINS: &[&str] = &["cd"];
+const BUILTINS: &[&str] = &["exit", "cd"];
 
 pub fn is_builtin(keyword: &str) -> bool {
     BUILTINS.contains(&keyword)
@@ -9,6 +10,7 @@ pub fn is_builtin(keyword: &str) -> bool {
 
 pub fn execute_builtin(keyword: &str, args: &[&str]) -> Result<(), ShellError> {
     match keyword {
+        "exit" => exit::run()?,
         "cd" => cd::run(args)?,
         _ => {}
     }