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/ast/compiler/tco.rs

@@ -40,6 +40,15 @@ impl TCO {
                 args: Box::new(Self::transform(Rc::new((**args).clone()), false)),
             },
 
+            BoundKind::Again { args } => {
+                if !is_tail_position {
+                    panic!("'again' is only allowed in tail position to avoid dead code.");
+                }
+                BoundKind::Again {
+                    args: Box::new(Self::transform(Rc::new((**args).clone()), false)),
+                }
+            }
+
             BoundKind::If {
                 cond,
                 then_br,