Yes, it's called a union type. What's shown on 133 is an example of that (though it's only used on that line). If we needed that type elsewhere, we could define it separately and reference it within the ScatterTrace type like this:

type Mode = 'lines' | 'markers' | 'lines+markers' | 'none' | 'text' | 'lines+text' | 'markers+text' | 'lines+markers+text';

export interface ScatterTrace extends TraceBase {
    ...,
    mode: Mode;
    ...,
}