feat: string-based-union support

[ThijsBroersen]

Initial draft of string based literal type union support.

[lihaoyi]

I think the approach looks fine as a first draft, but my concern is how it scales to broader usages of union types? Apart from unions of literal String types, can we also support unions of different primitives (e.g. String | Int | Boolean), or unions of different distinct case classes or objects?

I mainly to make sure there's some coherent approach that we can document and users can understand, and that whatever we implement now doesn't get in the way of implementing other union types in future since we'll be locking in the public API

[ThijsBroersen]

See #682 for my idea on how this could be done.