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/vm.rs

@@ -266,22 +266,27 @@ impl VM {
         self.resolve_tail_calls(observer, result)
     }
 
-    pub fn run_with_observer<O: VMObserver>(
-        &mut self,
-        observer: &mut O,
-        root: &ExecNode,
-    ) -> Result<Value, String> {
-        self.stack.clear();
+    pub fn eval_in_frame<O: VMObserver>(&mut self, observer: &mut O, root: &ExecNode, stack: Vec<Value>) -> Result<Value, String> {
+        self.stack = stack;
         self.frames.clear();
-
-        self.stack.resize(root.ty.stack_size as usize, Value::Void);
-
         self.frames.push(CallFrame {
             stack_base: 0,
             closure: None,
         });
         let result = self.eval_internal(observer, root);
         self.frames.pop();
+        result
+    }
+
+    pub fn run_with_observer<O: VMObserver>(
+        &mut self,
+        observer: &mut O,
+        root: &ExecNode,
+    ) -> Result<Value, String> {
+        let mut stack = Vec::new();
+        stack.resize(root.ty.stack_size as usize, Value::Void);
+
+        let result = self.eval_in_frame(observer, root, stack);
         self.resolve_tail_calls(observer, result)
     }
 
@@ -438,7 +443,7 @@ impl VM {
                     Ok(Value::Void)
                 }
             }
-            NodeKind::Block { exprs } => {
+            NodeKind::Block { exprs } | NodeKind::Program { exprs } => {
                 let mut last = Value::Void;
                 for e in exprs {
                     last = self.eval_internal(obs, e)?;