Add support for the `resume(value)` expression in effect handler
bodies. When resume is called, the value becomes the return value
of the effect operation, allowing handlers to provide values back
to the calling code.
Implementation:
- Add Resume(Value) variant to EvalResult
- Add in_handler_depth tracking to Interpreter
- Update Expr::Resume evaluation to return Resume when in handler
- Handle Resume results in handle_effect to use as return value
- Add 2 tests for resumable handlers
Example usage:
```lux
handler prettyLogger: Logger {
fn log(level, msg) = {
Console.print("[" + level + "] " + msg)
resume(()) // Return Unit to the call site
}
}
```
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
