Introduce boxing for captured variables

The binder now uses an `UpvalueAnalyzer` to identify local variables
that are captured by nested functions. These identified variables are
marked with `is_boxed: true` during `DefLocal` node creation. The VM
then uses this flag to wrap such variables in a `Value::Cell` (using
`Rc<RefCell<_>>`) to ensure they can be mutated across function calls.
feat: Introduce boxing for captured variables

Add UpvalueAnalyzer to identify variables captured by nested lambdas.
Modify Binder to use the analyzer and mark captured local variables for
boxing.
Update BoundKind::DefLocal to include an `is_boxed` flag.
Update VM to box captured variables when they are defined.
Add tests for upvalue capture detection.

src/ast/compiler/tco.rs

@@ -98,6 +98,12 @@ impl TCO {
                     ..node
                 }
             },
+            BoundKind::DefLocal { slot, value, is_boxed } => {
+                 Node {
+                    kind: BoundKind::DefLocal { slot, value: Box::new(Self::transform(*value, false)), is_boxed },
+                    ..node
+                }
+            },
             BoundKind::DefGlobal { global_index, value } => {
                  Node {
                     kind: BoundKind::DefGlobal { global_index, value: Box::new(Self::transform(*value, false)) },