Support export destructured assignment

Choose one: is this a bug report or feature request?
Feature request

@GeoffreyBooth I came across an unsupported `export` syntax that appears to be standard
### Input Code

```coffee
export {foo, bar} = item
```

### Expected Behavior

Should transpile to `export var {foo, bar} = item`

### Current Behavior

Doesn't compile

### Possible Solution

Guessing it won't be bad to add this as a supported grammar rule

### Context

Not blocking anything, just came across it while working on AST stuff

### Environment

* CoffeeScript version:
* Node.js version:

Reproduce the CoffeeScript input `export {foo, bar} = item` with the compiler and compare it with the expected `export var {foo, bar} = item` output. Start by tracing the grammar and AST handling for export and destructured assignment; done means the syntax compiles to the expected JavaScript without regressing related cases.

Written by the indexing model from the issue text.