Add Program node kind

The `Program` node kind is introduced to represent top-level code
blocks, distinguishing them from `Block` nodes which introduce new
scopes. This commit updates various compiler passes to handle the
`Program` node, ensuring correct AST traversal and processing.

The `Program` node is similar to `Block`, but its definitions propagate
to the enclosing scope, unlike `Block` which creates a new, isolated
scope. This distinction is important for how variables and functions are
resolved within the compiled code.

src/ast/compiler/lambda_collector.rs

@@ -22,7 +22,7 @@ where
 
     fn visit(&mut self, node: &Node<P>) {
         match &node.kind {
-            NodeKind::Block { exprs } => {
+            NodeKind::Block { exprs } | NodeKind::Program { exprs } => {
                 for expr in exprs {
                     self.visit(expr);
                 }