Add a syntactic action that takes JSON pasted into a Swift file or
placed in a string literal, then turns it into a set of Codable
structs that can represent the JSON. Our typical example starts like
this:

```
{
    "name": "Produce",
    "shelves": [
        {
            "name": "Discount Produce",
            "product": {
                "name": "Banana",
                "points": 200,
                "description": "A banana that's perfectly ripe."
            }
        }
    ]
}
```

and turns into this:

```swift
struct JSONValue: Codable {
    var name: String
    var shelves: [Shelves]

    struct Shelves: Codable {
        var name: String
        var product: Product

        struct Product: Codable {
            var description: String
            var name: String
            var points: Double
        }
    }
}
```

When converting to JSON, we attempt to reason about multiple JSON
objects on the same level to detect when there are optional fields,
due to either an explicit null or due to the absence of fields in some
of the JSON objects that are conceptually stored together.

The refactoring itself would live down in the swift-syntax package if
not for its dependency on Foundation. We'll move it when appropriate.