Add 'again' keyword for recursive calls

The `again` keyword is introduced to facilitate explicit recursive
function calls.
It is restricted to tail-call positions to prevent dead code and ensure
TCO
optimization. Type checking is enhanced to validate argument types
against
function parameters.

src/integration_test.rs

@@ -263,6 +263,15 @@ mod tests {
         }
     }
 
+    #[test]
+    #[should_panic(expected = "'again' is only allowed in tail position to avoid dead code.")]
+    fn test_again_non_tail_panic() {
+        let env = Environment::new();
+        let source = "(do (def f (fn [x] (do (again [(- x 1)]) x))) (f 5))";
+        // This will trigger the TCO pass which contains the validation logic
+        let _ = env.run_script(source);
+    }
+
     #[test]
     fn test_dynamic_call_destructuring_underflow() {
         let env = Environment::new();