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/mod.rs

@@ -1,7 +1,9 @@
 pub mod binder;
 pub mod bound_nodes;
 pub mod tco;
+pub mod upvalues;
 
 pub use binder::*;
 pub use bound_nodes::*;
 pub use tco::*;
+pub use upvalues::*;