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/environment.rs

@@ -3,7 +3,8 @@ use crate::ast::compiler::binder::{Binder, CompilerScope, LocalInfo};
 use crate::ast::compiler::{CapturePass, TypeChecker, TypedNode};
 use crate::ast::nodes::{AnalyzedPhase, Symbol, SyntaxKind, SyntaxNode};
 use crate::ast::parser::Parser;
-use crate::ast::vm::{Closure, TracingObserver, VM};
+use crate::ast::closure::Closure;
+use crate::ast::vm::{TracingObserver, VM};
 use std::cell::RefCell;
 use std::collections::{HashMap, HashSet};
 use std::path::{Path, PathBuf};