Introduce Value::Cell for mutable upvalues, allowing closures to

modify captured variables.
Implement `capture_upvalue` and `get_value`/`set_value` methods in the
`VM` to handle these mutable upvalues.

Refactor number parsing to support integers and floats

Separate the `TokenKind::Number` into `TokenKind::Integer` and
`TokenKind::Float`.
Update the lexer to distinguish between integer and float literals.
Update the parser to correctly map these new token kinds to their
respective `Value` types.

Add integration tests for parsing integers and floats, and for closure
modification of captured variables.

src/ast/parser.rs

@@ -51,7 +51,8 @@ impl<'a> Parser<'a> {
         let identity = Arc::new(NodeIdentity { location: token.location });
 
         let kind = match token.kind {
-            TokenKind::Number(n) => UntypedKind::Constant(Value::Float(n)),
+            TokenKind::Integer(n) => UntypedKind::Constant(Value::Int(n)),
+            TokenKind::Float(n) => UntypedKind::Constant(Value::Float(n)),
             TokenKind::String(s) => UntypedKind::Constant(Value::Text(s)),
             TokenKind::Keyword(k) => UntypedKind::Constant(Value::Keyword(Keyword::intern(&k))),
             TokenKind::Identifier(id) => {