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/binder.rs

@@ -289,6 +289,19 @@ impl Binder {
                 ))
             }
 
+            UntypedKind::Again { args } => {
+                if self.functions.len() <= 1 {
+                    return Err("'again' is only allowed inside a function or lambda.".to_string());
+                }
+                let args = self.bind(args)?;
+                Ok(self.make_node(
+                    node.identity.clone(),
+                    BoundKind::Again {
+                        args: Box::new(args),
+                    },
+                ))
+            }
+
             UntypedKind::Block { exprs } => {
                 let mut bound_exprs = Vec::new();
                 for expr in exprs {