Introduce closure AST node

The `Closure` struct has been moved from `ast/vm.rs` to a new module
`ast/closure.rs`. This improves the organization of the AST nodes. The
`Closure` struct represents a compiled function body along with its
captured upvalues, which is a key feature for Myc Script.

src/ast/mod.rs

@@ -1,3 +1,4 @@
+pub mod closure;
 pub mod compiler;
 pub mod diagnostics;
 pub mod environment;