I'm not sure why `let` in a `for` loop needs special parsing handling. There may not be clarity yet on what the parse tree should be, but I don't think it'll even remotely end up being like a transpilation as we explored.

I imagine the parse tree will be just like a normal `for` loop, except:

```
"declarations": [
    {
        "type": "VariableLetDeclarator",  // <--- special type to indicate 
        "id": {
            "type": "Identifier",
            "name": "i"
        },
        "init": {
            "type": "Literal",
            "value": 0,
            "raw": "0"
        }
    }
]
```
It doesn't seem like this should be mixed up with any discussion about how to handle the Firefox-specific `let (..) { .. }` syntax.