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/analyzer.rs

@@ -123,6 +123,15 @@ impl<'a> Analyzer<'a> {
                 (BoundKind::Call { callee: Box::new(callee_m), args: Box::new(args_m) }, p)
             }
 
+            BoundKind::Again { args } => {
+                let args_m = self.visit(Rc::new((**args).clone()));
+                if let Some(lambda_id) = self.lambda_stack.last() {
+                    self.recursive_identities.insert(lambda_id.clone());
+                    is_recursive = true;
+                }
+                (BoundKind::Again { args: Box::new(args_m) }, Purity::Impure)
+            }
+
             BoundKind::Block { exprs } => {
                 let mut new_exprs = Vec::with_capacity(exprs.len());
                 let mut p = Purity::Pure;