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/parser.rs

@@ -140,6 +140,7 @@ impl<'a> Parser<'a> {
             match sym.name.as_ref() {
                 "if" => self.parse_if(identity),
                 "fn" => self.parse_fn(identity),
+                "again" => self.parse_again(identity),
                 "def" => self.parse_def(identity),
                 "assign" => self.parse_assign(identity),
                 "do" => self.parse_do(identity),
@@ -154,6 +155,15 @@ impl<'a> Parser<'a> {
         result
     }
 
+    fn parse_again(&mut self, identity: Identity) -> Result<Node<UntypedKind>, String> {
+        let args = Box::new(self.parse_expression()?);
+        Ok(Node {
+            identity,
+            kind: UntypedKind::Again { args },
+            ty: (),
+        })
+    }
+
     fn parse_if(&mut self, identity: Identity) -> Result<Node<UntypedKind>, String> {
         let cond = Box::new(self.parse_expression()?);
         let then_br = Box::new(self.parse_expression()?);