Refactor: Remove explicit upvalue analysis pass

The `UpvalueAnalyzer` pass is no longer necessary as the binder now
directly tracks captures. This commit removes the `UpvalueAnalyzer`
struct and associated logic from `src/ast/compiler/upvalues.rs`.

The `Binder::bind_root` function now returns the `captures` map, which
is then processed by a new `CapturePass` in `src/ast/environment.rs`
before type checking. This consolidates capture logic within the binder
and its subsequent processing steps.

src/ast/compiler/mod.rs

@@ -1,6 +1,7 @@
 pub mod analyzer;
 pub mod binder;
 pub mod bound_nodes;
+pub mod captures;
 pub mod dumper;
 pub mod lambda_collector;
 pub mod macros;
@@ -8,14 +9,13 @@ pub mod optimizer;
 pub mod specializer;
 pub mod tco;
 pub mod type_checker;
-pub mod upvalues;
 
 pub use binder::*;
 pub use bound_nodes::*;
+pub use captures::*;
 pub use dumper::*;
 pub use macros::*;
 pub use optimizer::*;
 pub use specializer::*;
 pub use tco::*;
 pub use type_checker::*;
-pub use upvalues::*;