This adds an `appendInterpolation` overload to
`DefaultStringInterpolation` that includes a parameter for providing a
default string when the value to interpolate is `nil`. This allows this
kind of usage:

```swift
let age: Int? = nil
print("Your age is \(age, default: "timeless")")
// Prints "Your age is timeless"
```

The change includes an additional fixit when optional values are
interpolated, with a suggestion to use this `default:` parameter.