Allow collapsing definitions

[eira-fransham]

Probably this would just be something like turning the line object into a

enum SourceElement {
    Line { .. },
    Scope {
        /// The text shown when this scope is collapsed
        /// For this enum, it would be `enum SourceElement`
        summary: String,
        elements: Vec<SourceElement>,
    }
}

and handling this in the JS. I don't know how we'd actually generate that though.