Implement a Formula Parser / Evaluator

Question

## Problem

Parse and evaluate a simple formula with +, -, *, /, parentheses, and cell references — Coda blends docs with a spreadsheet/formula layer.

## Example
```
"=2*(A1+3)" with A1=4 -> 14
```

## Constraints
- Valid expression; handle precedence

## Approach
Tokenize -> shunting-yard or recursive descent -> evaluate. Resolve cell refs.