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/main.rs

@@ -2,6 +2,8 @@ use eframe::egui;
 use crate::ast::environment::Environment;
 
 pub mod ast;
+#[cfg(test)]
+mod integration_test;
 
 fn main() -> eframe::Result {
     let options = eframe::NativeOptions {