Implement a Safe Expression Evaluator

Question

## Problem

Implement a small, safe evaluator for user-provided expressions (e.g., `{{ user.name }}` or arithmetic) without using eval — Retool's reactive bindings.

## Example
```
eval("{{a}} + {{b}}", {a:2,b:3}) -> 5
```

## Constraints
- No arbitrary code execution; handle bad input

## What Retool looks for
Reported Retool style. Tokenize/parse and evaluate against a scope; reason about sandboxing.