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/nodes.rs

@@ -86,6 +86,9 @@ pub enum UntypedKind {
         callee: Box<Node<UntypedKind>>,
         args: Box<Node<UntypedKind>>,
     },
+    Again {
+        args: Box<Node<UntypedKind>>,
+    },
     Block {
         exprs: Vec<Node<UntypedKind>>,
     },